I think you are right. I came across a website that uses WP Ultimo -
https://enzaime.com/ . In the registration process, the last step is Custom Domain and there I can search and register a custom domain for my website (see the attachment).
So I think I need to have a registration on OpenSRS to be able to provide custom domains on the registration process. Also, the description of the plugin WP Ultimo: Domain Seller is: "Sell
custom domains to your customers right from the registration screen! Supports OpenSRS for now, with more options to come."