The prefix for HTTP Query tags is HTTP.
{%%http.First%%}
HTTP variables are optionally passed as part of the URL. For example:
http://www.mycompany.com/team/Profile.html?First=John&Last=Smith
{%%HTTP.First%%} would produce John, and {%%HTTP.Last%%} would produce Smith. Any name not found in the URL query list, will be replaced with an empty string.