Programmatically creating UIButton & set the constraints in code. Setting the button in middle of the screen for box x&y axis.

UIButton_Programmatically_Constraints

let takeVideoButton = UIButton(type: .system)
//takeVideoButton.frame = CGRect(x: 30, y: 30, width: 100, height: 50)
takeVideoButton.setTitle("Take Video", for: .normal)
takeVideoButton.addTarget(self, action: #selector(takeVideo), for: .touchUpInside)
self.view.addSubview(takeVideoButton)

takeVideoButton.translatesAutoresizingMaskIntoConstraints = false
let horizontalConstraint = NSLayoutConstraint(item: takeVideoButton, attribute: .centerX, relatedBy: .equal, toItem: self.view, attribute: .centerX, multiplier: 1, constant: 0)
let verticalConstraint = NSLayoutConstraint(item: takeVideoButton, attribute: .centerY, relatedBy: .equal, toItem: self.view, attribute: .centerY, multiplier: 1, constant: 0)
let widthConstraint = NSLayoutConstraint(item: takeVideoButton, attribute: .width, relatedBy: .equal, toItem: self.view, attribute: .width, multiplier: 1, constant: 100)
let heightConstraint = NSLayoutConstraint(item: takeVideoButton, attribute: .height, relatedBy: .equal, toItem: self.view, attribute: .height, multiplier: 1, constant: 100)
NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])