Sports Add On with Google Play

I live in the Netherlands and have an HBO Max subscription with a 'lifetime discount'. This means the standard subscription (ad-free) for €5.99 instead of €11.99. Billing is handled via Google Play. I want the sports add-on. I cannot add this because billing is done through Google Play. I contacted HBO, and they advised me to cancel my subscription and take out a new one directly with HBO. However, that would invalidate my discount. That isn't really the solution I want. Does anyone have any other tips?