Add this to a MonoBehaviour to set its Script Execution Order without access to the editor.

SetScriptExecutionOrder.cs

#if UNITY_EDITOR
    private const int executionOrder = -1000;

    [UnityEditor.InitializeOnLoadMethod]
    private static void SetScriptOrder()
    {
        var go = new GameObject("Temp");
        var monoScript = UnityEditor.MonoScript.FromMonoBehaviour(go.AddComponent<NAME_OF_THIS_MONOBEHAVIOUR>());
        if (UnityEditor.MonoImporter.GetExecutionOrder(monoScript) != executionOrder)
        {
            UnityEditor.MonoImporter.SetExecutionOrder(monoScript, executionOrder);
        }
        DestroyImmediate(go);
    }
#endif