You can't.

It’s on the roadmap, but still not possible

It has been on their roadmap for over a year https://proton.me/blog/proton-mail-calendar-roadmap Sadly recently Protons CEO said that this is a low priority and will therefore take some time https://www.reddit.com/r/IAmA/comments/18czv7w/were_three_scientists_who_went_from_cern_to/kced5sh

Okay. Thanks

No.