In the Agency Settings you will see two tabs the General tab and Redemption tab. 

In the General tab you have two text boxes, one for your Agency API key and second for Custom Domain.

For the API key, you will have to go into your highlevel account, generate an API key at the agency level and then you will paste it in the API key text box.

For the Custom Domain, this will allow you to use your own domain on redemption pages, coupon pages etc. As all of the pages are currently using the ghlpromo.com domain. In order for your custom domain to work you will have to point the domain to our IP_ADDRESS (34.231.44.18) in your DNS settings. Once you have that done and click on save settings, wait for a couple of hours for the custom domain to be resolved and then it should work.