Custom Control - Reacting to Events outside of a control class

I'm creating a custom control for windows 10. I know that I can attach events to the TemplateChildren by using GetTemplateChild("Button") and then I can assign a method for example to the Tapped Event.

Now I want to assign an Method to an event outside the Control .cs class. In Mainpage.xaml I can assign e.g. a Method to the Tapped event like this: <local:MyControl Tapped="New_Hanlder"><local:MyControl>.

But how can I assign a method to a child of that control and not to the whole? Like for example I got a custom control with a grid and an Button, how can I only handle the Button Click Event in the Mainpage.xaml.cs file and NOT in the control file?

I tried this in my Control .cs class:

AppBarButton b;

In the method which is loaded when the Control is loaded:

b = GetTemplateChild("Go") as AppBarButton;

And then I got this public method:

   public AppBarButton getAppBarButton()
    {
        retu b;
    }

But accessing the AppBarButton is throwing an Exception.

Please help me to get this working. Thanks in regard!