PBXProject.BuildConfigByName

Возвращает GUID конфигурации сборки с указанным именем для конкретной цели.

Возвращает Null, если конфигурация не существует.

using UnityEditor;
using System.IO;
using UnityEditor.Callbacks;
using UnityEditor.iOS.Xcode;

public class Sample_BuildConfigByName  
{
    [PostProcessBuild]
    public static void OnPostprocessBuild(BuildTarget buildTarget, string pathToBuiltProject)
    {

        // Stop processing if build target is not iOS
        if (buildTarget != BuildTarget.iOS)
            return;

        // Initialize PBXProject
        string projectPath = PBXProject.GetPBXProjectPath(pathToBuiltProject);
        PBXProject pbxProject = new PBXProject();
        pbxProject.ReadFromFile(projectPath);

        // Get main target GUID
        string mainTargetGuid = pbxProject.GetUnityMainTargetGuid();

        // Get the GUID of the `Debug` config so you can use it to change config-specific settings
        string configGuid = pbxProject.BuildConfigByName(mainTargetGuid, "Debug");

        // For example, add a Build Property
        pbxProject.AddBuildPropertyForConfig(configGuid, "exampleProperty", "exampleValue");

        // Apply changes to the pbxproj file
        pbxProject.WriteToFile(projectPath);
    }
}