You want to make it into a “Track Matte” — a quick google search yielded the following:
https://www.editorsguild.com/v2/magazine/Newsletter/MarApr03/tip_ae_track_mattes.html
https://help.adobe.com/en_US/AfterEffects/9.0/WS3878526689cb91655866c1103906c6dea-7cf9a.html
Ben Unguren
Motion Graphics & Editing
http://www.mostlydocumentary.com
Some contents or functionalities here are not available due to your cookie preferences!
This happens because the functionality/content marked as “Vimeo framework” uses cookies that you choosed to keep disabled. In order to view this content or use this functionality, please enable cookies: click here to open your cookie preferences.