In order to use your Protonmail account with Postbox, you need to have the Protonmail Bridge app installed on your system.
In Postbox navigate to the File menu and select New > Mail Account. Provide your Protonmail email and password generated by the Protonmail Bridge app.
The password to be used for the connection to the Bridge is generated by the Bridge when the Protonmail account is created. To view that password/key, open the account info dialog on the Bridge.
Supplying that password key and using default settings detected by Postbox will allow for a successful connection to the Protonmail bridge and the mail server.