If your registered email is in the Apple ID format (e.g. ...@icloud.com), you can use Apple sign-in. Otherwise, use the general sign-in.
1.
Go to the PhoneBox website or open up the link: PhoneBox Sign in
2.
Click <Sign in with Apple>
3.
Fill it in with your Apple ID email address and password.
4.
Click - the < Continue > button on the page.
5.
Then you will be able to log into the PhoneBox self-serve account.