Preferred practice is to drop “http://”, “https://” or “www” in URL (web address) references.
Most web servers will accept URLs with or without “www” and redirect as necessary, or the “www” prefix may conflict with another subdomain (example: alumnus.tennessee.edu).
There may be exceptions in cases when the URL won’t work for certain browsers if the “www” is not inserted as part of the address.
It is recommended that the URL be tested and confirmed—including whether the use of “www” is necessary—before it is included in written communication. Treat each case as practical considerations demand.