httpservice.lua

local url_query = {arg1="val1", arg2="val2"}
local post_json = {a=1, b=2, this="is json"}

local HttpService = game:GetService("HttpService")

local options = {
  Url = ("https://httpbin.org/post?%s=%s&%s=%s"):format("arg1", url_query.arg1, "arg2", url_query.arg2),
  Method = "POST",
  Headers = {["Content-Type"] = "application/json"},
  Body = HttpService:JSONEncode(post_json)
}

local r = HttpService:RequestAsync(options)

print(r.Headers["content-type"])
local data = HttpService:JSONEncode(r.Body)

requests.lua

local url_query = {arg1="val1", arg2="val2"}
local post_json = {a=1, b=2, this="is json"}

local http = require(ReplicatedStorage.http)

local r = http.post("https://httpbin.org/post", { query=url_query, data=post_json })

print(r.headers["content-type"])
local data = r:json()