http://protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7ozyd.onion/blog/what-is-email-alias
You don’t need to take any extra steps to receive mail using a Proton Mail +alias. Just think of a new address with a plus in the format above and give it out. Any mail sent to that address will arrive in your usual Proton Mail inbox. Unlike most other email services, when you reply to mail sent to a +alias in Proton Mail, the alias automatically appears as the default From address.