How to pass json value to API using post in C #?

I am new to C #, you can tell me the best way to pass json API value in C # using POST.

Below I have written pyhton code but dont know how to do it in C #.

string payload = {
   "username" : "1432143352759a61bca0ad4a35a4f0", # when you register, they'll return this
   "password" : "password",
   "client_id" : "c3ca1565b0dbedd74c5c",
   "client_secret" : "e0591fe41c4e2585b313ad93e2748022db3186d0",
}

res = requests.post("https://sandbox.synapsepay.com/api/v2/user/login", payload)
print res.status_code
print res.text