How can I add a bus and tram p to my oyster card?

I know how to top up my oyster card with pay as you go, and it's clear how to add a tube p to my oyster card, but I can't find anywhere to add a bus p to my oyster card. The oyster top up machines seem to only offer tube pes, normal tickets or pay as you go. I've registered to be able to top up my oyster online, but all it will let me add is credit or a tube p.