<TextBox ... >
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="GotFocus">
            <i:InvokeCommandAction Command="{Binding cmd_GotFocusTextBox}" />
        </i:EventTrigger>
    </i:Interaction.Triggers>
</TextBox>

''' <summary>
    ''' Variable für das RelayCommand cmd_GotFocusTextBox
    ''' </summary>
    ''' <remarks></remarks>
    Private _cmd_GotFocusTextBox As RelayCommand

    ''' <summary>
    ''' Command für GotFocusTextBox
    ''' </summary>
    ''' <value></value>
    ''' <returns></returns>
    ''' <remarks></remarks>
    Public ReadOnly Property cmd_GotFocusTextBox As RelayCommand
        Get
            If (_cmd_GotFocusTextBox Is Nothing) Then
                _cmd_GotFocusTextBox = New RelayCommand(AddressOf TextLeer)
            End If
            Return _cmd_GotFocusTextBox
        End Get
    End Property
 
    ''' <summary>
    ''' Funktion die bei cmd_GotFocusTextBox ausgeführt werden soll
    ''' </summary>
    ''' <remarks></remarks>
    Public Sub TextLeer()

    End Sub