Call parent component from children user control

Hello, I am trying to call a combobox located in a usercontrol from another usercontrol.

Parent user control:


               




                           
                               
                              


   


Child user control (ucCM.xaml):

I have tried with this code, but it isn't working because "this.Parent" returns null.

ComboBox cmbA = (ComboBox)((this.Parent) as UserControl).FindName("cmbA");