You're using GZDoom Builder, right? When setting the action special and arguments, choose action 80 (ACS_Execute). There should be a check box for named scripts. Tick it, and choose the name of the script from the drop-down list (you might need to compile the ACS code before it can be seen in the list).
That's as far as I recall on how to do it. I don't have GZDoom Builder installed, and it's been like two years or so since the last time I used it, so things could be different.