You just have to load the script as one of your input scripts the same way control scripts for movement are loaded.
From the menu tree on the left, go to scripts. On the right hand part of the screen find the script for the StartButton (it’s in the user scripts I believe. Double click it to add it to your input scripts. Then assign the input (Select at start screen, null, press start button, startbutton script).
I’m not at my pc but, that should be what you’re looking for