Since you already have a company account on Project Desk, you should have received a invitation e-mail with instructions to validate your e-mail address. If you did not receive the e-mail, please make sure your e-mail address was correctly entered by the account administrator.

It is the account administrator who sends out invitations e-mails to join Project Desk. You can ask the account administrator to go into the "Manage Users" screen and re-send the invitation.