Sending Push Notification from backend Server and the XML to include Toast Launch Attributes

I'm using Service bus to send Windows Push notifications from my backend service and send Notification as below:

String.Format("<?xml version=\"1.0\" encoding=\"utf-8\"?>" + "<toast>" +"<visual>" + "<binding template=\"ToastImageAndText03\">" + "<image id=\"1\" src=\"{0}\" alt=\"{1}\" />" +"<text id=\"1\">{2}</text>" +"<text id=\"2\">{3}</text>" +"</binding>" +"</visual>" +"</toast>", imageUrl, fullName, headlineText, bodyText);

Now, it sends Notifications successfully. I want to add is

((XmlElement)toastNode).SetAttribute("launch", "{\"type\":\"toast\",\"param1\":\"12345\",\"param2\":\"67890\"}");

Can someone please explain how to actually achieve the above adding to my original Notification Payload.