How to add back button to the content page navigation bar

Hi, I am new to forms My question is how to add back button to the content page navigation bar. My Main page is like
MainPage = new NavigationPage(new LoginPage()); .
My LoginPage is content page. I want to add back button to the loginpage navigation bar. I using NavigationPage.SetHasBackButton(this, true); but it not showing any back button. And also how to send the data form page2 to page1 by using Navigation.PopModalAsync();

Thank you.