post https://secure.nmi.com/api/v4/usernames/search
Never Use Real API Keys
Never use real API Keys when testing. The gateway allows Partners to create Test Merchant Accounts. Testing should always use keys from the Test Accounts and never keys from a Standard Account.
Usage
This is optional, but you can check to make sure a username is available before submitting your merchant creation request.This can be useful if you want to do form validation on your website to prevent the user from entering a username that has already been taken.