If you just signed up, use the default password with your requested email, and input that into mail.p0ntus.com. Change your password in the interface, and follow the next steps using your email and password.
To sign in, use user.p0ntus.com either on the login page with the "Webmail" button, or in the signed-in interface, with the "Sign in to webmail" button.
You also may have to enter your 2FA code:
Once you are signed in, you can use the webmail to send and receive emails. You can also use the calendar and contacts features.
When you are done, click the "Exit" icon in the top right corner of the UI.