A sample of programmatically assign constraints to a UIButton in Swift.

buttonConstraint.swift

import UIKit

class ViewController: UIViewController {
  
  struct LayoutProperties {
    static let sideSpacing: CGFloat = 20
    static let cornerRadius: CGFloat = 8
    static let buttonHeight: CGFloat = 60
  }
  
  private let subscribeButton: UIButton = {
    let b = UIButton(type: .system)
    b.translatesAutoresizingMaskIntoConstraints = false
    b.setTitle("MY FAVORITE BUTTON", for: .normal)
    return b
  }()
  
  override func viewDidLoad() {
    super.viewDidLoad()
    view.addSubview(subscribeButton)
    subscribeButton.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: LayoutProperties.sideSpacing).isActive = true
    subscribeButton.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
    subscribeButton.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    subscribeButton.heightAnchor.constraint(equalToConstant: LayoutProperties.buttonHeight).isActive = true
    
    styleUIComponents()
  }
  
  private func styleUIComponents() {
    subscribeButton.layer.cornerRadius = LayoutProperties.cornerRadius
    subscribeButton.backgroundColor = UIColor.red
    subscribeButton.setTitleColor(.white, for: .normal)
  }
}