Steps:
1. Add UI Button to the Hierarchy
2. Make a prefab out of it
3. Select instance in the Hierarchy
4. click "+" to add listener for OnClick function of the Button
==>
"Runtime only" is highlighted as property modification
5. Right-click on it and choose "Apply to Prefab Button"
==>
Two exceptions are thrown:
Destination property could not be found
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PrefabUtility.ApplySingleProperty (UnityEditor.SerializedProperty instanceProperty, UnityEditor.SerializedObject prefabSourceSerializedObject, System.String assetPath, System.Boolean

Note: Assigned GameObject to field will not be applied to prefab asset as well
Workaround: You can apply modification through Modified component or via Overrides menu