XF 2.0 How can I access the value of Username for custom user field validation on registration page.

[Sam96]

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 is valid or not during registration. Any ideas how this can be accomplished?

 

[S Thomas]

$user_name = array_values($_POST)[1];