Can I use my pre-existing HBO Max account on a Fire Stick?

I have tried Googling and searching this subreddit to no avail— I'm staying with my boyfriend for a few months and I wanted to watch HBO Max through my account on his Fire stick. But the only option the app gives me is "start free trial." Can I sign into an existing account?? He and I have literally searched all over the app and we can't figure it out. He went ahead and just started a free trial, but I'm kind of annoyed.