Acutally we are not able to register Bulletproof Domains for you. You have to bring your own Domain which is registred somewhere else!
Open your Website plan from the Clientarea here: https://www.ionhost.io/clientarea.php?action=services
At the left side of the Navigation you will find the URL "Login to cPanel":
There you can find the IP Address (v4) of your hosting plan:
You have to point your Domain/Subdomain Records to this IP Address - also you can use this IP for your FTP/SFTP connection.
To use Subdomains or Domain Aliases you have to configure them in your cPanel.