Example of how to send sms in Xamarin.Forms Applications - http://julianocustodio.com/sms-xamarin-forms

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:DemoSMS"
             x:Class="DemoSMS.MainPage">

    <StackLayout VerticalOptions="StartAndExpand">
        <Label Text="Number:"/>
        <Entry x:Name="Number"  Keyboard="Telephone"/>
        <Label Text="Message:"/>
        <Entry x:Name="Message" />
        <Button Text="SEND" Clicked="Send"/>
    </StackLayout>

</ContentPage>