<i:Interaction.Behaviors>
    <beh:EventToCommandBehavior Command="{Binding DropCommand}" Event="Drop" PassArguments="True" />
</i:Interaction.Behaviors>

public ActionCommand<DragEventArgs> DropCommand { get; private set; }

this.DropCommand = new ActionCommand<DragEventArgs>(OnDrop);

  private void OnDrop(DragEventArgs e)
    {
        // ...
    }