Pair my Website Domain to another platform that designs websites

I am creating a brand new website and have bought the domain which is a .[login to view URL] domain and am using a United States based website design platform. I am having difficulty pairing the two together so I can create my website through the US based design [login to view URL] , and using the .[login to view URL] address for my main domain. For someone with intermediate knowledge it shouldn't be difficult. I have included the error messages I receive from the website I bought the domain from "Free Parking". I'm looking for people that have experience setting up websites as well, maybe web design, for an ongoing maintenance, or the one-off question that I have that I could pay hourly. In a nutshell, this is a small project, but could turn into a website maintenance relationship down the line. Advanced level of English preferred as I am based in New Zealand and need to be able to communicate effectively as sometimes website design can be tricky.