You can connect an external domain to any upgraded Bookmark site in your account. The first thing you will need to do is upgrade the site to either a Professional or Business plan. You can view the details and pricing of the plans here; https://www.bookmark.com/pricing
Once that is done, you will need to point the domain to Bookmark by altering the domain's DNS records with your domain registrar. You can follow the steps on how to change the CNAME and A record to point to Bookmark here; https://help.bookmark.com/en/articles/2637832-how-to-connect-a-custom-domain-purchased-outside-of-bookmark
Lastly, when the domain has been pointed you can go into the website builder to Site > Domain inside the sidebar and connect the external domain, and publish your site to see the domain connected.