OverrideModuleUsernameGeneration What's the hook code?

[dpgthing]

How do I set the product username equal to that of the client's email address upon the client clicking "Complete Order"?

I create a custom product field requiring the client to input an email address.  How do I translate this field to replace the Product username field?

I've been directed to use:

<?php
add_hook('OverrideModuleUsernameGeneration', 1, function($vars) {
    // Perform hook code here...
});

What goes in "perform hook code here..." to accomplish what I described above?  I'm a complete noob when it comes to php script code.