Go to System Preferences>Internet Accounts.
At the top of the page, it says "Internet Accounts sets up your accounts to use with Mail, Contacts, Calendar, Messages, and other apps."
You have choices like Google, Yahoo. Now if Proton Mail were one of them, I would not have any problems. I would not have asked this question. I could simply click on the choice. This System Pref proves that it is possible to link Mail or Calendar to internet accounts. One need only uncover a way.
Apple, Google, and Yahoo all have on-line calendars that can be accessed via a web browser. When you select Calendar on one of those email accounts it links the Calendar app on your Mac to the on-line calendar provided by Google, Yahoo,
etc. If Protonmail had an on-line calendar then it might be possible to link Apple's Calendar app with that on-line calendar, but as far as I can find protonmail does not offer that service.
Unlike appointment
invitations Apple's Calendar app sends appointment
reminders as
Notifications and there is no provision for sending them via text message or email. I don't know if Yahoo or Google's on-line calendars can send reminders via Email or not but if they did it would be through their own SMTP server and even if they could be configured to use protonmail's SMTP server there could be no end to end encryption because the encryption takes place in the protonmail bridge app on your MacOS or iOS device.
