xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"

xmlns:mv="http://www.galasoft.ch/mvvmlight"

xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions"

<controls:BindableRichTextBox x:Name="rtb" Document="{Binding ShowSendMessage,UpdateSourceTrigger=PropertyChanged}" FontSize="20" Grid.Row="1" Background="#FFF9F9F9" Focusable="True" Margin="10,0" BorderThickness="0" AcceptsReturn="False">

    <i:Interaction.Triggers>

        <ei:KeyTrigger FiredOn="KeyUp" Key="Enter">

            <mv:EventToCommand Command="{Binding SendCommand}"/>

        </ei:KeyTrigger>

    </i:Interaction.Triggers>

</controls:BindableRichTextBox>