how do i add beat headphones to my account

Which exact model of Beats "headphones" do you have? AFAIK, only the Beats earbuds can be associated with the Find My service, which in turn, would be associated with your Apple ID.

Ref: https://www.beatsbydre.com/stories/2021/05/find-my-keep-track-of-your-beats

Ok, those would be Beats Studio Buds

Ref: Set up and use your Beats Studio Buds or your Beats Studio Buds + - Apple Support

What I'm not 100% sure on, as I don't have any Beats products, is whether or not, they will show up as a device under your Apple ID ... even though they can participate in Find My. Maybe, someone else here who does, will chime in with the answer.