You can get the authcode for your domain in your Control Panel.
Please log into your account and click on the menu item [Domains] . You can now see a list of all your Domains.
On the right side next to your Domain, click [Manage] (pen icon).
Scroll down to the bottom and click on the [Show Authcode] button. In the final step, you can just copy the authcode and hand it over to your new Provider.