ios – How to add shadow to border?

Basically shadow applies to the whole layer not just the border. You may need to try like below:-

import UIKit

class GlowButton: UIButton {

    private let gradientLayer = CAGradientLayer()

    override init(frame: CGRect) {
        super.init(frame: frame)

        setupUI()
    }

    required init?(coder: NSCoder) {
        super.init(coder: coder)

        setupUI()
    }

    private func setup() {
        setTitle("▶ Start watching your Channel’s here", for: .normal)
        setTitleColor(.white, for: .normal)
        titleLabel?.font = UIFont.boldSystemFont(ofSize: 20)
        layer.cornerRadius = 16
        clipsToBounds = false
        layer.borderWidth = 2
        layer.borderColor = UIColor(red: 1, green: 0.2, blue: 0.4, alpha: 1).cgColor
        layer.shadowColor = UIColor.red.cgColor
        layer.shadowOpacity = 0.9
        layer.shadowRadius = 15
        layer.shadowOffset = .zero
        gradientLayer.colors = [
            UIColor(red: 0.5, green: 0, blue: 0.1, alpha: 1).cgColor,
            UIColor(red: 1, green: 0, blue: 0.2, alpha: 1).cgColor
        ]

        gradientLayer.startPoint = CGPoint(x: 0, y: 0.5)
        gradientLayer.endPoint = CGPoint(x: 1, y: 0.5)

        gradientLayer.cornerRadius = 16

        layer.insertSublayer(gradientLayer, at: 0)
    }

    override func layoutSubviews() {
        super.layoutSubviews()

        gradientLayer.frame = bounds
    }
}