iRule to change a take a URL and add URI then send to a Pool.

Try this one:

    when HTTP_REQUEST {
      switch [string tolower [HTTP::host]] {
        "sharepoint.test-site.com" {
          if { [HTTP::uri] equals "/" } {
           HTTP::redirect "https://[HTTP::host]/Pages/default.aspx"
          }
          else {
           pool /Internal/F5-Internal-Pool_sharepoint
          }
        }
        default {
          discard
        }
      }
    }