How to add custom call in an animation?

Hi,

I would like to call a function at a certain point. In the "custom" field - it reads "Add any CSS or GSAP property".

If I do so, console log shows

Invalid property gsap.call set to ap_removeElement("#ap_after_body_tag") Missing plugin? gsap.registerPlugin()

Is the call "gsap.call" correct or do I need to pick the ID of the animation e.g. "_mp_1639327237"?

The "ap_removeElement" function exists.

Thank you.