Is it a girlfriend or a female friend? Cause these type of magic things could get really cringy or awkward depending on your relationship. That being said, I'd consider some balloon effects, those are overused at promposals so you could have fun changing up something overused there. There's a color-changing T-shirt thing you can buy here on theory11 that could work for the reveal/promposal, fun things you could do with loops/invisible string for a levitation effect of the four letter's "PROM", but something I personally would do is just visit your magic repertoire and see what you can turn into a promposal rather than try to come up something completely original. A lot of basic magic effects can be modified to fit something else.
Just some advice for whatever promposal you do, don't make it obvious that you're about to do it. The magic + the surprise of a promposal hits really hard and could literally melt minds. However, if it's obvious the promposal is coming, some of the magic could be lost because they're waiting for that promposal and are expecting that any minute and not focusing on the effect at hand.
For my junior prom (I'm a senior in high school currently), I did a variation on Asi Wind's "Double Exposure" which is a card trick so if that turns you off, stop reading here. I think it goes really well with a promposal because there's surprise, she's not expecting a promposal in the moment, and she's left with a tangible impossible souvenir on her phone. It's also a different kind of card trick than the one's people are used to so it had a different feel than those "4 Jacks Rob a Bank" type of tricks.