How do I implement a variable into a GUI?

I'm trying to make a leveling system that lets you expend your XP to level up. The higher level you are, the stronger you'll be due to things like permanent potion effects.

I have a custom slot-less GUI hooked up to a keybind that will let me see three variables: the player's level, XP, and XP needed for the next level. 

The only problem is that I cannot figure out how to input the actual variables into the textfield. I saw some comments about returning the variables via procedures, but nothing seems to work. Every time I write a procedure, the option in the textfield remains 'FIXED.'

How can I resolve this?