Hi -
I'm really an audio guy myself, but as I understand it you'd accomplish this by creating a (Bezier) mask (Draw Mask), and then updating the mask periodically using keyframes, making the mask follow the motion of the object.
I've attached a link to a FCP course on FX (at the top) -- there's a section on working with masks, and the following section goes over animation -- check out video 18 on creating the Mask and the next few videos that follow (particularly video 20 and on) on using keyframes to animate effects. Hopefully there'll be something helpful in there.
Perhaps these Apple support pages on animating masks in FCP might also be of some help:
https://support.apple.com/guide/final-cut-pro/animate-a-mask-vercef2b6821/mac
https://support.apple.com/guide/final-cut-pro/animate-an-effect-shape-mask-verd1ddfe518/mac
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