As in, when you write a function with variables you use as input. There should be a way option to have it prompt you for custom input for those variables.
Then you would select the option to "Run some Code" and enter whatever you want, and when you press the button it will be evaluated as if you put it in the console. If you need something more advanced than one line, I recommend using my nifty Server Command GUI mod to write a longer script.
Just a question,
would you be able to make the background of the overlay invisible?
I have it set so that you can adjust the color of the background but not the transparency, but that is a pretty simply change that I can do. Right now, if you set the animation ticks preference to exactly 1, the background becomes invisible for reasons I am not sure of and will probably be fixed next patch.