How to access CodeActivity In Argument Value in Code Behind

Question

Can Someone please help me how to access Code Activity's InArgument in CodeBehind of xaml designer

I am trying to play around with Activity Designer for understanding From other posts on stack overflow I learned to use ModelItem is used for accessing argument values of CodeActivity in Xaml

But How to access the values in code behind ?

    [Designer(typeof(ActivityDesigner1))]
public class Class1 : CodeActivity
{   
public InArgument<String> FirstName { get; set; }
protected override void Execute(CodeActivityContext context)
{
    // Activity Action Logic
}
}

Here is the Activity Designer XAML

<sap:ActivityDesigner.Resources>
    <ResourceDictionary>
        <sapc:ArgumentToExpressionConverter x:Key="ArgumentToExpressionConverter" />
    </ResourceDictionary>
</sap:ActivityDesigner.Resources>
<Grid x:Name="MyGrid">
    <Grid.RowDefinitions>
        <RowDefinition Height="50">
            </RowDefinition>
        <RowDefinition Height="50"></RowDefinition>
    </Grid.RowDefinitions>
    <TextBox x:Name="MyBtn" Grid.Row="0" Text = "{Binding Path=ModelItem.FirstName, 
Converter={StaticResource ArgumentToExpressionConverter},ConverterParameter=In, 
Mode=TwoWay}" TextChanged="OnTextChange" />
</Grid>

And Here is the code Behind

public partial class ActivityDesigner1
{
    public ActivityDesigner1()
    {   
        InitializeComponent();
    }
    private void OnTextChange(object sender, RoutedEventArgs e)
    {
     // WANT TO ACCESS FirstName  HERE
    }


}