Can I only use Apple to pay for HBO plus or can I use my credit card?

If you subscribe through Apple it uses whatever payment method you have on file for your Apple ID account, which by definition would be your credit card or even Pay Pal if you have it setup.

Link-> Change, add, or remove Apple ID payment methods - Apple Support

Link-> Payment methods that you can use with your Apple ID - Apple Support

If you start the subscription directly with HBO at their website, then you can use whatever method you want there too outside of your Apple ID account.

HBOMax Billing:

https://help.hbomax.com/us/Answer/Detail/000001195