You could use the following code snippet:
Uri myurl = new Uri(Request.QueryString["SPHostUrl"]);
string param1 = HttpUtility.ParseQueryString(myUri.Query).Get("p");
or use
string param1 = Request.QueryString["p"];
If you want to this via JS, then go on with this
function getQueryStringParameter(paramToRetrieve) {
var params;
var strParams;
params = document.URL.split("?")[1].split("&");
strParams = "";
for (var i = 0; i < params.length; i = i + 1) {
var singleParam = params[i].split("=");
if (singleParam[0] == paramToRetrieve)
return singleParam[1];
}
}
var sProp = decodeURIComponent(getQueryStringParameter("StringProperty1"));
document.write('Value of StringProperty1 : ' + sProp + '</br>');