let stackView = UIStackView()
let subView = UIView()
subView.backgroundColor = .red
subView.translatesAutoresizingMaskIntoConstraints = false
stackView.addSubview(subView) // Important: addSubview() not addArrangedSubview()
// use whatever constraint method you like to
// constrain subView to the size of stackView.
subView.topAnchor.constraint(equalTo: stackView.topAnchor).isActive = true
subView.bottomAnchor.constraint(equalTo: stackView.bottomAnchor).isActive = true
subView.leftAnchor.constraint(equalTo: stackView.leftAnchor).isActive = true
subView.rightAnchor.constraint(equalTo: stackView.rightAnchor).isActive = true
// now add your arranged subViews...
stackView.addArrangedSubview(button1)
stackView.addArrangedSubview(button2)