I'm curious how? This FAQ page on their site claims you cannot search email contents, although they plan to support that in their upcoming Protonmail Bridge (which works with a desktop client to search locally): https://protonmail.com/support/knowledge-base/search/

That is what I'd expect, since obviously they cannot build a search index over text they cannot read. Transferring the entire contents of your emails to your web browser to search locally would be slow and impractical on anything but a very fast connection.