I was able to get it to work using this:

let scene = UIApplication.shared.connectedScenes.first
if let sd : SceneDelegate = (scene?.delegate as? SceneDelegate) {
    sd.blah()
}

If your app will only ever have one scene then I was able to get it to work using this:

let scene = UIApplication.shared.connectedScenes.first
if let sd : SceneDelegate = (scene?.delegate as? SceneDelegate) {
    sd.blah()
}