If you bought a domain from another provider, you're more than welcome to use a sub-domain of that domain with our service. You simply need to create that sub-domain through your registrar and then point it to our 18.104.22.168 IP Address. If you don't know how to do that or what this means, then you should ask your registrar to set this up for you. We are unable to help point the sub-domain here.
Ultimately, you'll be creating a record with your domain registrar that looks something like this one:
Once the sub-domain is pointed here, you need to assign it to your site. Edit the site, go to the Settings tab and select the Change Site Address link.
Then enter the sub-domain you've pointed here in the Use a Domain You Already Own field.
Then Save and Publish your site. It may take your registrar up to 48 hours to point the domain here, so please keep that in mind. The sub-domain may not start working with your site immediately.