How can I display a custom variable as a text in an overlay?

So i'm trying to make an overlay that display your current health all the time. I have the script set up and everything, the problem is, I couldn't figure out how to bind the string variable that is set to my current health to a text box in the overlay. Every time I open the text box that should display my health, it only allows the "Fixed" option, (as in it's not letting me use any variables), and nothing else. Sorry if that was hard to follow, I can elaborate in the comments if you want.