To use a Proton email account with an email application, you need to install the Bridge on your device fist. You can download it from the Proton website, but be aware that the Bridge is not available for free Proton users. That means if you have a free Proton account, you can’t use it with an email application. You will need to use their webmail interface instead.
If you have a paid subscription from Proton, install the Bridge, then follow my instructions here, Protonmail Bridge Setup