MainWindow.xaml

<Window x:Class="HowToCreateLookUpEdit.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525"
        xmlns:dxg="http://schemas.devexpress.com/winfx/2008/xaml/grid"
        xmlns:dxe="http://schemas.devexpress.com/winfx/2008/xaml/editors"
        xmlns:local="clr-namespace:HowToCreateLookUpEdit"
        xmlns:viewmodel="clr-namespace:HowToCreateLookUpEdit.ViewModel"
        xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm"
        xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core">
    <Window.DataContext>
        <viewmodel:ProductListViewModel/>
    </Window.DataContext>
    <Grid>
        <dxg:LookUpEdit Name="lookUpEdit1" ItemsSource="{Binding Path=Products}"
                        HorizontalAlignment="Center" VerticalAlignment="Center"
                        Width="200"
                        DisplayMember="ProductName"
                        ValueMember="ID" 
                        AddNewButtonPlacement="EditBox"
                        AutoComplete="True"
                        IncrementalFiltering="True"
                        AutoPopulateColumns="True"
                        ImmediatePopup="True"
                        IsPopupAutoWidth="False">
            <dxmvvm:Interaction.Triggers>
                <dxmvvm:EventToCommand PassEventArgsToCommand="True" Command="{Binding ShowProductFormCommand}" EventName="ProcessNewValue" SourceName="lookUpEdit1">
                    <dxmvvm:EventToCommand.EventArgsConverter>
                        <viewmodel:ProductListValueConverter/>
                    </dxmvvm:EventToCommand.EventArgsConverter>
                </dxmvvm:EventToCommand>
            </dxmvvm:Interaction.Triggers>
        </dxg:LookUpEdit>               
    </Grid>
    <dxmvvm:Interaction.Behaviors>
        <dx:DialogService DialogWindowStartupLocation="CenterOwner">
            <dx:DialogService.ViewTemplate>
                <DataTemplate>
                    <local:UserControl1/>
                </DataTemplate>
            </dx:DialogService.ViewTemplate>
            <dx:DialogService.DialogStyle>
                <Style TargetType="dx:DXDialogWindow">
                    <Setter Property="Width" Value="345"/>
                    <Setter Property="Height" Value="324"/>
                </Style>
            </dx:DialogService.DialogStyle>
        </dx:DialogService>
    </dxmvvm:Interaction.Behaviors>
</Window>