Proton currently does not accept Monero as a payment method.

David Peterson (@davidgpeterson on X), who is the General Manager of Proton VPN, has stated they will add Monero support to their Summer roadmap if there is an online petition with 1000 people acknowledging they'd switch to Proton VPN (Or a Proton product in general)

(Screenshot from https://x.com/davidgpeterson/status/1939019623237570951)

By signing this petition, you are acknowledging you will start using or continue to use Proton VPN (or another Proton product broadly) on the condition that they will accept Monero as a payment method for their services.