Create GUI Texture without drag&drop(programatically)

Hi,
Yes, I know that GUI textures are deprecated but I'm working on an old project and when I import the project into Unity 2017.3, some links to GUI textures break and I need to add them manually.
So I have a Prefab which has a GUI texture field like this:

Code (CSharp):

1. public GUITexture menu;

which could be set by drag&drop in the editor if the prefab was added to a scene by drag&drop. But, I create the Prefab programatically(the Prefab is loaded with Resources.Load(path)). My question is how can I set the GUI texture programatically because drag&drop does not work on a non-scene object/texture?