You are entitled to one custom domain. If you get the domain when your trial is over, it's free. To attach a domain while you are in trial mode there is a small fee
Go to /admin/profile page and click Attach custom domain button. You will be directed to the page where you can enter a domain to see if it's available
If the domain you selected is available, you will be directed to the "Domain available screen". Click "Get domain" to proceed.
If you are in the trial period you will be presented with a credit card form.
Once you submit payment if required you will receive confirmation page
upon completion your custom domain will be attached to your rentalsetup account. It takes about 1hr-3 days for it to be recognized though.