[0] https://api.protonmail.ch/pks/lookup?op=get&search=solarleak...
But I don't think the full API is documented. There are some attempts to reverse engineer an API from the WebClient [1], but they tend to be... Brittle.
[0] https://protonmail.com/support/knowledge-base/download-publi...
[1] https://github.com/ProtonMail/WebClient
[0] https://api.protonmail.ch/pks/lookup?op=get&search=solarleak...