You need a reference to your MainWindow. You can get the reference through Application.Current.MainWindow. All you have to do is cast it to MainWindow type:
var canvas = ((MainWindow)Application.Current.MainWindow).Get_Canvas;
However I don't think that accesing controls from another type is a good idea. You should reconsider why you need a reference to 'myCanvas' from another type. There is a reason why objects instantiated in xaml are private.
Hope that helps.