Issue in login logout page navigation due to Hardware back button

I have created a simple login and registration app consuming webApi services and it is developed using MVVM model. I have not written a single line of code in somepage.xaml.cs file. After the user logs into the system the he is navigated to homepage but if i press the hardware back button it again goes to login page.
I have searched a bit on this issue and found out OnBackButtonPressed(). It has to be used in xaml.cs . But instead of hitting OnBackButtonPressed() on hardware hit it is not hitting.
It is instead hitting on the toolbar item logout button.