I realize this is a bit late, but you would generally accomplish this using Unity's AssetBundles. There's a script in the KCToolkit (github repo linked below) that exports any set of Unity Objects (Prefabs, Audio, Meshes, anything in the Assets folder essentially) to an AssetBundle that can be extracted by a mod, and injected into the game. AssetBundles are used because Unity is weird and doesn't like to just have things work directly, there's always a process to follow. Once you've created your AssetBundle, you need to write code that uses its contents. How you would do it is basically the same way you would handle Prefabs in the Unity Engine. You extract the AssetBundle's contents as shown below by scene path (from the Unity Editor) and that should return a UnityEngine.Object (which can be cast to a GameObject). This object that is returned is the prefab of your canvas/UI, this can be instantiated however you like, for a reference I recommend looking at Zat's ModMenu (also linked below).


Kingdoms and Castles Toolkit:
https://github.com/mpeddicord/Kingdoms-and-Castles-Toolkit

Zat's ModMenu:
https://github.com/BigMo/KCMods/tree/master/Scripts/ModMenu

A further in-depth explanation is featured in the section of the modding tutorial, Making a Mod with AssetBundles (Ogre Sneakers):
https://docs.google.com/document/d/1hRrV92_n9zuYxuB-7yCaiftzFsGEsoRRkMnTbaaKRqM/edit#heading=h.q3co8lyn7pc6