You should not use this utility! The GNU Privacy Guard (gpg(1)) is capable of
retrieving PGP keys from Protonmail using WKD [0]. This also works for some
other addresses.

Here’s how to do it, using my e-mail address as an example:
$ gpg --auto-key-locate wkd --locate-keys emma@tebibyte.media

And to import it:
$ gpg --auto-key-locate wkd --auto-key-import --locate-keys emma@tebibyte.media

[0] https://proton.me/blog/security-updates-2019