Type in your web domain with the extension/webmail, e.g. www,yourdomain name.com/webmail.
When the prompt comes up type your full email address (firstname.lastname@example.org) in the user field, and your password (for that email account) in the password field.
You are then taken to a mail reader selection screen where you choose which email you want to use. It doesn’t matter which one you pick, though Round Cube is the newest.