Declaring your converter

App.xaml

<?xml version="1.0" encoding="utf-8" ?>
<Application xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:converter="clr-namespace:MyApp.Converter;assembly=MyApp"
             x:Class="MyApp.App">
    <Application.Resources>

        <ResourceDictionary>
            <converter:ItemTappedEventArgsConverter x:Key="ItemTappedEventArgsConverter" />
            <converter:FirstValidationErrorConverter x:Key="FirstValidationErrorConverter" />
            <converter:BooleanNegationConverter x:Key="Inverter" />
        </ResourceDictionary>
    </Application.Resources>
</Application>