Yes, you can use an external domain that you have already secured. The following are the edits you need to make to your host records with your Registrar account.
In the HOST/DNS Records settings, add the following CNAME and A records:
Please also make sure to remove any other records that aren't needed, disable any forwarding and make sure your Name Servers are on their default settings.
Let us know when this is done by writing to firstname.lastname@example.org and we can integrate from our end. Please do include your custom domain name and your Appy Access Code. It's up to two business days for us to integrate thereafter.