Example how your Xamarin.Forms application get the IMEI code from device - http://julianocustodio.com/imei

MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:DemoImei"
             x:Class="DemoImei.MainPage">

    <StackLayout HorizontalOptions="Center"
                 VerticalOptions="CenterAndExpand">
        <Label x:Name="LblImei" />
        <Button Text="Get Imei" Clicked="Button_OnClicked"/>
    </StackLayout>

</ContentPage>