Adding button to end game screen

[RogueHammer]

I feel like a right idiot but I'm struggling to do something as simple as adding an extra button to the Victory/Defeat screen.
Is there a guide to do something this simple?
And am I meant to create new art assets to add a new button? Because I can't get the Asset Editor to load. (missing Civ6.Art.xml)
Do I have to edit the Controls.blp?

 

[RogueHammer]

In the LUa.log its says "attempt to index a nil value stack traceback:" Clearly I have forgotten to add something somewhere.