Overlay text label can't change

I am making an experience system for a Minecraft mod, and I am trying to figure out how to display different numbers using overlay text boxes. I tried making a procedure that just has "Return number: Get number (global: ForageXP) for entity: (Event/Target Entity)" but it did not show up as an option in the value of the label editor for an overlay, all it said was "Fixed:". how do I make it so it displays the value of the variable I want it to?