Assuming you haven’t yet got a domain set up in Helm, you’ll need to click the “Domains” button to get started:

Click “Add New” and you will be prompted to choose your registration method.

Select the type appropriate to you (if unsure, ask your web host) and click “Next”.
Enter your domain name, then click “Save”. You will now see your domain added.
Clicking this domain will take you to the Domain Menu. However, you may first want to make sure your domain is registered.