This depends if you have created obj1 from the JSON, or if you are trying to create the JSON from an object you have instantiated in C#.
First case, you simply access the request members by:
obj1.request.latitude = "33.75";
obj1.request.longitude = "-84.39"
Second case:
obj1.request = new () {
latitude = "33.75",
longitude = "-84.39",
};