I already have HBOMax that I am paying for. I have the opportunity to switch to HBOMax free of charge with my new AT&T fiber installation.

HBO has a pretty good Help Center.

I think this link is what you want...

https://help.hbomax.com/us/Answer/Detail/000001204

If I was in this situation I'd probably chicken out for fear I'd screw something up and spend days chasing down billing issues so I'd just use one of the contact methods here....

https://help.hbomax.com/us/ContactUs