There is no direct way how to do this "automatically". Usually, there is no need to store it in C++ structure too.
I usually do something like
Value r = ParseJSON(json_text)["response"];
r["username"]
r["email"]
Note that things are aranged so that failed ParseJSON returns empty Value and trying to access key in empty Value or missing key results in another empty Value (so the code above does not crash if json is invalid or keys are missing, just results in blanks..)