https://github.com/dotnet/roslyn/issues/57347

GetAppHostPath.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Build" Version="16.11.0" ExcludeAssets="runtime" />
    <PackageReference Include="Microsoft.Build.Locator" Version="1.4.1" />
  </ItemGroup>

</Project>

Program.cs

using System;
using System.IO;
using System.Linq;
using System.Xml;
using Microsoft.Build.Evaluation;
using Microsoft.Build.Locator;

// Note: code using MSBuild has to be in another method than the call to MSBuildLocator
MSBuildLocator.RegisterDefaults();

string GetAppHostPath()
{
  var xml = @"
<Project Sdk=""Microsoft.NET.Sdk"">
    <PropertyGroup>
      <OutputType>Exe</OutputType>
      <TargetFramework>net5.0</TargetFramework>
    </PropertyGroup>
</Project>";

  var project = Project.FromXmlReader(XmlReader.Create(new StringReader(xml)), options: new()).CreateProjectInstance();

  project.Build("ProcessFrameworkReferences", loggers: null);

  return project.GetItems("AppHostPack").Single().GetMetadata("Path").EvaluatedValue;
}

Console.WriteLine(GetAppHostPath());