If you want to host a website on your own domain name with us, simply create the hosting account first, add your domain name to it, and then setup your website on your domain name.
Setting up the website on the free subdomain first and moving it to your own domain afterward is definitely an option, but it’s more work.
So yes, there is no option to immediately create your hosting account with a custom domain. But by setting up your website on the “wrong” domain, you can assume that’s going to cause more trouble down the line.
You can see the directories your domain names are assigned to in the client area. And you can move your websites from one directory to another like this: