Haveing problem with webmail

[#1]

I started with Plusnet fibre on Monday, contacted customer support this morning to set up an email address. I was given the address mail@username.plus.com and I picked a simple password which I would change once I logged into webmail. I have tried all combinations of the email address and I cannot login to webmail, I can login to the username mailbox using the username and password I provided at sign up, Where am I going wrong?
Thanks,
Gerald

[#2]

To log in to an additional mailbox you have to use the format of username+mailbox for the ID part of the login and just to remind you the corresponding password for the mailbox.
The login doesn't accept email addresses for the ID.

[#3]

Many thanks,
Gerald