I tried to find instructions for this but most were from a few years ago. These may not be exact but it will get you to the right place.
1. Go to your Dashboard.
2. Click on Character.
3. On the Character page, click the ADD CHARACTER button.
4. Select and enter the info as prompted.
5. You will then be directed to login to the Loadstone for a verification code. Follow those instructions to complete the setup.