In the original setup I already explain the configuration, to do the same with Protonmail will need to install Proton Mail Bridge(https://proton.me/mail/bridge), then with configuration as below.
[sendemail] smtpEncryption = STARTTLS smtpServer = 127.0.0.1 smtpUser = <your id>@protonmail.com smtpServerPort = 1025 smtpPass = <your password>