Open AppStore through button

Here. But I highly suggest you learn the basics of Swift!

UIApplication.sharedApplication().openURL(NSURL(string: "itms://itunes.apple.com/de/app/x-gift/id839686104?mt=8&uo=4")!)

If you wanna open the AppStore in Swift 5:

if let url = URL(string: "itms-apps://itunes.apple.com/app/id1629135515") {
    UIApplication.shared.open(url)
}