Gets the page request group identifier, a page request group is the id used to associate the source page
that was used to download scripts as well as post-backs by JavaScript against API endpoints.
Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2