username validation

I'm using the following method to validate the custom user field. https://xenforo.com/community/resources/custom-user-field-callback-validate-value.379/ However, for my situation, I need to access the Username that the user typed into the registration form to determine if the custom user field...

The registration form has a username validation function. it seems this can be abused by attackers to find out what accounts are present on the site. The attacker can run a script to try millions of possible usernames to get a member list. I have recently encountered such attack on my vbulletin...