Select the DNS Zone Editor in the Domains section.
Select the domain you wish to modify from the drop down menu.
Scroll down to the section Add a DNS Record.
For the Host Record, enter only the subdomain of the address you want to use for your Unbounce custom Domain. For example, if you pickedpromos.mydomain.com as your address, enter promos here. Or, if you're using a root domain such as www.mydomain.com then you'll want to enterwww here. Do not leave this blank.
Select CNAME from the Type dropdown menu.
Enter unbouncepages.com as the Points To value.
Click Add Record Now. You may need to wait up to a couple of hours for the CNAME to function.