Swift 2.2 on iOS, 380 bytes
let r = NSMutableURLRequest(URL:NSURL(string:"http://codegolf.stackexchange.com/posts/84546/comments")!)
r.HTTPMethod = "POST"
r.HTTPBody = try!NSJSONSerialization.dataWithJSONObject(["comment":"1234567890123456","fkey":UIWebView().stringByEvaluatingJavaScriptFromString("localstorage.getItem('se:fkey')")!],options:[])
NSURLSession.sharedSession().dataTaskWithRequest(r).resume()
Assumes the user is logged in to Code Golf with Safari and has cookies enabled. Also assumes UIKit is implicitly imported and available. JSON serialization is so verbose in Cocoa... Might update this with a Swift 3 solution and optionally macOS/Linux testable answers.