The check new user name behavior will only work with Dreamweavers Insert record server behavior. If you use the DataAssist insert record server behavior instead, you would have to first set it up using the DW Insert, apply the check new user name behavior. A hidden form element is used for triggering the check new user, you need to copy that, then remove the DW Insert , apply the DA Insert then past the hidden form element back for the check new user behavior to function.
after doing this, the check new user behavior will no longer be editable.
It can be done either way, but for compatibility with the DA Insert, it's a bit easier to use the security assist method instead.