There is no search for the forums. I don't see the need for one anyway, the vast majority of posts are just pointless rants and arguments.
If you're looking for your latest posts, you can find them at https://www.bungie.net/7/en/User/Profile/2/4611686018458829304?bgn=Powan
If you're looking for news and the latest Destiny 2 info, go to the below link. https://www.bungie.net/7/en/News