Adding a button to the title bar Xamarin Forms

Use ToolbarItem from YourPage.xaml.cs:

ToolbarItems.Add(new ToolbarItem("Search", "search.png",() =>
{
  //logic code goes here
}));

In Xaml:

<ContentPage.ToolbarItems> 
    <ToolbarItem Icon="search.png" Text="Search" Clicked="Search_Clicked"/>
</ContentPage.ToolbarItems>

Update

Xamarin.Forms introduced TitleView in 3.2.0 that you can customize title of the navigation.

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="TestPage">
    <NavigationPage.TitleView>
        <Button ... />
    </NavigationPage.TitleView>
    ...
</ContentPage>