How can i access objects from code behind?

I have created a new ContentPage in Xamarin.Forms. What I want to do is layout my contentpage in xaml. So if I put a button on the xaml, I want to access the button in the .cs code behind.

I am trying to do :

But when I run I get a compile error saying : Property content is null or is not IEnumerable in InitializeComponent()

Any help?