I guess you have worded the question incorrectly. You cannot redirect to a master page, or even request for it for that matter.
I think your question is about how to change the master page at runtime.. If that is the question, it can be done in Page_PreInit event life cycle of the page.
Define the master page in the content page’s Page_PreInit method. Page_PreInit is the last opportunity you have to override the default master page setting, because later handlers (such as Page_Init) reference the master page. For example, the following code defines the master page based on the Session object.
void Page_PreInit(Object sender, EventArgs e)
{
if (Session["masterpage"] != null)
MasterPageFile = (String)Session["masterpage"];
}
The MasterPageFile property can be set with the path of the master page.