The answer
Find the domain setup instructions and exact DNS values in your website service. Depending on the service, you may need A, AAAA or CNAME records. DNS connects the name to its destination; it does not upload website files.
What to do
Add the domain to the website service first.
With default nameservers, enter the supplied values in your domain’s DNS tab.
Configure the root domain and www as instructed; retain existing mail records.
After the change, check the website and valid HTTPS on both addresses.