The Auth code (Authorisation code) for your domain name is normally used to transfer Registrars for your domain name.
Here's how to request your Auth code:
1) Login to the 1st Domains Account Manager and select 'Manage Domains & Services'
2) Select the domain name you wish to manage from the list
3) Under the 'Authorisation Key (UDAI/EPP)' section, click 'Request Key' and an email will be sent to the registrant's email address with the Authorisation code information.