string Strings = "http://alianzas.qapaq.pe/PlataformaGestion/Gestion/?ref=Affiliate_AskRobin&VariableR=853199";
int indice1 = strings.IndexOf("&VariableR=") + "&VariableR=".Length;
//Resultado me sale 85

Uri myUri = new Uri("http://www.example.com?param1=good&param2=bad");
string param1 = HttpUtility.ParseQueryString(myUri.Query).Get("param1");