If I’m not mistaken, you’re referring to the Branding feature of CrossBox? If so, you should be able to set it up by creating an admin[@]domain[.]com account, logging into this account, and navigating to Admin > Branding.
However, I am having issues using it since it throws errors as it requires that the custom mail[.]domain[.]com CNAME (for SMTP/IMAP/POP3) points to mail[.]mxlogin[.]com, instead of servername[.]mxlogin[.]com as instructed in the official documentation of MXRoute.
I did test trying to set up mail2[.]domain[.]com CNAME mail[.]mxlogin[.]com and I was able to successfully use http://webmail.domain.com; I’m not sure, however, if it’s a valid custom hostname to use. @Jarland?