This code is used to inject a callback into Unity's internal build system. They should have this but if you really want to you can just make your own. This only needs Mono.Cecil #Magic

Injector.cs

    public class Injector
    {
        [MenuItem("Hacks/Inject Build Callback")]
        public static void BuildCallback()
        {
            Assembly.BeginEditingAssembly(AssemblyTypes.UnityEditor, editAlreadyModifed:true);
            {
                TypeDefinition buildPiplelineType = Assembly.GetType<BuildPipeline>();
                MethodDefinition buildInternalMethod = buildPiplelineType.GetMethod("BuildPlayerInternal");

                TypeReference MultiCastDelegateTypeRef = Assembly.Import<System.MulticastDelegate>();
                TypeDefinition MultiCastDelegateTypeDef = new TypeDefinition("", "OnBuildStartedDelegate", TypeAttributes.Public, MultiCastDelegateTypeRef);
                MethodDefinition ConstructorMethodDef = new MethodDefinition(".ctor", MethodAttributes.Public | 
                                                                                       MethodAttributes.CompilerControlled | 
                                                                                       MethodAttributes.RTSpecialName | 
                                                                                       MethodAttributes.SpecialName | 
                                                                                       MethodAttributes.HideBySig,
                                                                                       Assembly.Import(typeof(void)));
                // Constructor
                ConstructorMethodDef.IsRuntime = true;
                ConstructorMethodDef.HasThis = true;
                ConstructorMethodDef.IsHideBySig = true;
                ConstructorMethodDef.IsPublic = true;
                ConstructorMethodDef.Parameters.Add(new ParameterDefinition("levels", ParameterAttributes.None, Assembly.Import<string[]>()));
                ConstructorMethodDef.Parameters.Add(new ParameterDefinition("buildLocation", ParameterAttributes.None, Assembly.Import<string>()));
                ConstructorMethodDef.Parameters.Add(new ParameterDefinition("buildTarget", ParameterAttributes.None, Assembly.Import<BuildTarget>()));
                ConstructorMethodDef.Parameters.Add(new ParameterDefinition("buildOptions", ParameterAttributes.None, Assembly.Import<BuildOptions>()));
                MultiCastDelegateTypeDef.Methods.Add(ConstructorMethodDef);

                // Invoking Method
                MethodDefinition BeginInvokeMethodDef = new MethodDefinition("BeginInvoke", MethodAttributes.Public |
                                                                                       MethodAttributes.HideBySig | 
                                                                                       MethodAttributes.NewSlot | 
                                                                                       MethodAttributes.Virtual, 
                                                                                       Assembly.Import<System.AsyncCallback>());
                BeginInvokeMethodDef.IsRuntime = true;
                BeginInvokeMethodDef.HasThis = true;
                BeginInvokeMethodDef.IsHideBySig = true;
                BeginInvokeMethodDef.IsRuntimeSpecialName = true;
                BeginInvokeMethodDef.IsSpecialName = true;
                BeginInvokeMethodDef.IsPublic = true;
                BeginInvokeMethodDef.IsVirtual = true;
                BeginInvokeMethodDef.Parameters.Add(new ParameterDefinition("buildLocation", ParameterAttributes.None, Assembly.Import<string>()));
                BeginInvokeMethodDef.Parameters.Add(new ParameterDefinition("buildTarget", ParameterAttributes.None, Assembly.Import<BuildTarget>()));
                BeginInvokeMethodDef.Parameters.Add(new ParameterDefinition("buildOptions", ParameterAttributes.None, Assembly.Import<BuildOptions>()));
                BeginInvokeMethodDef.Parameters.Add(new ParameterDefinition("callback", ParameterAttributes.None, Assembly.Import<System.IAsyncResult>()));
                BeginInvokeMethodDef.Parameters.Add(new ParameterDefinition("object", ParameterAttributes.None, Assembly.Import<object>()));
                MultiCastDelegateTypeDef.Methods.Add(BeginInvokeMethodDef);


                // End Invoke
                MethodDefinition EndInvokeMethodDef = new MethodDefinition("EndInvoke", MethodAttributes.Public | MethodAttributes.HideBySig | MethodAttributes.NewSlot | MethodAttributes.Virtual, Assembly.Import(typeof(void)));
                EndInvokeMethodDef.Parameters.Add(new ParameterDefinition("result", ParameterAttributes.None, Assembly.Import<System.IAsyncResult>()));

                EndInvokeMethodDef.IsRuntime = true;
                EndInvokeMethodDef.HasThis = true;
                EndInvokeMethodDef.IsHideBySig = true;
                EndInvokeMethodDef.IsRuntimeSpecialName = true;
                EndInvokeMethodDef.IsSpecialName = true;
                EndInvokeMethodDef.IsPublic = true;
                MultiCastDelegateTypeDef.Methods.Add(EndInvokeMethodDef);


                // Invoke
                MethodDefinition InvokeMethodDef = new MethodDefinition("Invoke", MethodAttributes.Public | MethodAttributes.HideBySig | MethodAttributes.NewSlot | MethodAttributes.Virtual, Assembly.Import(typeof(void)));

                InvokeMethodDef.Parameters.Add(new ParameterDefinition("levels", ParameterAttributes.None, Assembly.Import<string[]>()));
                InvokeMethodDef.Parameters.Add(new ParameterDefinition("buildLocation", ParameterAttributes.None, Assembly.Import<string>()));
                InvokeMethodDef.Parameters.Add(new ParameterDefinition("buildTarget", ParameterAttributes.None, Assembly.Import<BuildTarget>()));
                InvokeMethodDef.Parameters.Add(new ParameterDefinition("buildOptions", ParameterAttributes.None, Assembly.Import<BuildOptions>()));

                InvokeMethodDef.IsRuntime = true;
                InvokeMethodDef.HasThis = true;
                InvokeMethodDef.IsHideBySig = true;
                InvokeMethodDef.IsVirtual = true;
                InvokeMethodDef.IsPublic = true;

                MultiCastDelegateTypeDef.Methods.Add(InvokeMethodDef);

                // Add our new delegate type
                buildPiplelineType.NestedTypes.Add(MultiCastDelegateTypeDef);
                // Add a field for it
                FieldDefinition delegateFieldDef = new FieldDefinition("onBuildStarted", FieldAttributes.Public | FieldAttributes.Static, MultiCastDelegateTypeDef.Resolve());
                buildPiplelineType.Fields.Add(delegateFieldDef);

                {
                    ILProcessor ilProcessor = buildInternalMethod.Body.GetILProcessor();
                    ilProcessor.InsertBefore(buildInternalMethod.Body.Instructions[0], ilProcessor.Create(OpCodes.Nop));
                    ilProcessor.InsertBefore(buildInternalMethod.Body.Instructions[0], ilProcessor.Create(OpCodes.Callvirt, InvokeMethodDef));
                    ilProcessor.InsertBefore(buildInternalMethod.Body.Instructions[0], ilProcessor.Create(OpCodes.Ldarg_3));
                    ilProcessor.InsertBefore(buildInternalMethod.Body.Instructions[0], ilProcessor.Create(OpCodes.Ldarg_2));
                    ilProcessor.InsertBefore(buildInternalMethod.Body.Instructions[0], ilProcessor.Create(OpCodes.Ldarg_1));
                    ilProcessor.InsertBefore(buildInternalMethod.Body.Instructions[0], ilProcessor.Create(OpCodes.Ldarg_0));
                    ilProcessor.InsertBefore(buildInternalMethod.Body.Instructions[0], ilProcessor.Create(OpCodes.Ldsfld, delegateFieldDef));
                    ilProcessor.InsertBefore(buildInternalMethod.Body.Instructions[0], ilProcessor.Create(OpCodes.Nop));
                }

                // Add invoke to BuildPlayerWindow
                TypeDefinition buildPlayerType = Assembly.GetType("UnityEditor", "BuildPlayerWindow");
                MethodDefinition buildDefaultMethodDef = buildPlayerType.GetMethod("BuildPlayerWithDefaultSettings", 3);

                {
                    UnityEngine.Debug.Log(buildDefaultMethodDef.Parameters[2].Name);

                    ILProcessor ilProcessor = buildDefaultMethodDef.Body.GetILProcessor();
                    int index = buildDefaultMethodDef.Body.Instructions.Count - 13;
                    ilProcessor.InsertBefore(buildDefaultMethodDef.Body.Instructions[index], ilProcessor.Create(OpCodes.Nop));
                    ilProcessor.InsertBefore(buildDefaultMethodDef.Body.Instructions[index], ilProcessor.Create(OpCodes.Callvirt, InvokeMethodDef));
                    ilProcessor.InsertBefore(buildDefaultMethodDef.Body.Instructions[index], ilProcessor.Create(OpCodes.Ldloc,  7)); // 7 Build Options 
                    ilProcessor.InsertBefore(buildDefaultMethodDef.Body.Instructions[index], ilProcessor.Create(OpCodes.Ldloc_1)); // Build Target
                    ilProcessor.InsertBefore(buildDefaultMethodDef.Body.Instructions[index], ilProcessor.Create(OpCodes.Ldloc, 5));  // 5 text
                    ilProcessor.InsertBefore(buildDefaultMethodDef.Body.Instructions[index], ilProcessor.Create(OpCodes.Ldloc, 14)); // 14 Levels
                    ilProcessor.InsertBefore(buildDefaultMethodDef.Body.Instructions[index], ilProcessor.Create(OpCodes.Ldsfld, delegateFieldDef));
                    ilProcessor.InsertBefore(buildDefaultMethodDef.Body.Instructions[index], ilProcessor.Create(OpCodes.Nop));
                }


            }
            Assembly.EndEditingAssembly(shouldSave: true);
        }
    }