UIViewController.swift

import UIKit
import PassKit
import Stripe

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
        addApplePayPaymentButtonToView()
    }

    private func addApplePayPaymentButtonToView() {
        let paymentButton = PKPaymentButton(paymentButtonType: .buy, paymentButtonStyle: .black)
        paymentButton.translatesAutoresizingMaskIntoConstraints = false
        paymentButton.addTarget(self, action: #selector(applePayButtonTapped(sender:)), for: .touchUpInside)
        view.addSubview(paymentButton)
        
        view.addConstraint(NSLayoutConstraint(item: paymentButton, attribute: .centerX, relatedBy: .equal, toItem: view, attribute: .centerX, multiplier: 1, constant: 0))
        view.addConstraint(NSLayoutConstraint(item: paymentButton, attribute: .centerY, relatedBy: .equal, toItem: view, attribute: .centerY, multiplier: 1, constant: 0))
    }

    @objc private func applePayButtonTapped(sender: UIButton) {
    }
}