Start by seeking out the hand-raisers - people who are already interested and comfortable doing video work. It might be someone who posts silly TikToks on their personal account. Then look for ways to incentivize their participation - free lunch, gift cards, swag, something fun.
If you don’t have anyone who naturally wants to do it and you’re having to do a little more coaxing, then do everything you can to reduce friction and anxiety in the experience. Overcommunicate the steps/process, if possible, do the filming yourself, give them a script and give them some time to go over it beforehand to get comfortable/natural with the lines and go out of your way to make them feel comfortable with the filming. If you’re asking them to film themselves on their phone and send that for a UGC-look, then give them super-detailed instructions on lighting and camera angle etc. with specific examples to learn from.