I am creating an registration dialog in swift with 3 text field and one Switch and I successfully add three text field two the Alert. The following code shows the same.
If you use Recycled Steel's answer above with iOS 13 you can use SF Symbols instead of PNGs. It will solve any scaling issues you might have.
checkbox.imageView.tintColor = UIColor.blackColor;
if (@available(iOS 13.0, *)) {
[checkbox setImage: [UIImage systemImageNamed:@"square"] forState: UIControlStateNormal];
[checkbox setImage: [UIImage systemImageNamed:@"checkmark.square"] forState: UIControlStateHighlighted];
[checkbox setImage: [UIImage systemImageNamed:@"checkmark.square"] forState: UIControlStateSelected];
}