How to pass a string from view to the viewmodel in the MVVM pattern?

The trick with MVVM is the View is a visual representation of the data. The ViewModel is the interface between the data and the view.

So for the Tabs to reflect the Button clicks, you should be binding the Tabs to the data and the Button clicks are bound to the Command that passes a CommandParameter to the ViewModel. Then the ViewModel updates the data and the binding will notify the View of the changes, then the View will refresh the UI.

I'm not quite sure which framework you are using or how you're wiring up your binding, but here is a minimal solution to do what you are asking.

So first we need to set up the Command to enable the Button in the View to pass data to the ViewModel:

public abstract class CommandBase : ICommand
{
    public event EventHandler? CanExecuteChanged;

    public virtual bool CanExecute(object? parameter) => true;

    public abstract void Execute(object? parameter);

    protected void OnCanExecuteChanged()
        => CanExecuteChanged?.Invoke(this, EventArgs.Empty);
}

public class ButtonClickCommand : CommandBase
{
    public ButtonClickCommand(Action<string> callback)
        => _callback = callback;

    private readonly Action<string>? _callback;

    public override void Execute(object? parameter)
        => _callback?.Invoke(parameter as string ?? string.Empty);
}

Next we need to set up the ViewModel to:
1. Expose an ObservableCollection for the Tabs
2. Expose and wire up then the Command for the Buttons
3. A method to update the Tabs collection with the new Tabs as the Buttons are clicked

public class MainViewModel
{
    public MainViewModel()
        => ButtonClickCommand = new ButtonClickCommand(OnClicked);

    public ICommand? ButtonClickCommand { get; }

    public ObservableCollection<string> Tabs { get; } = new();

    private void OnClicked(string buttonName)
        => Tabs.Add(buttonName);
}

Now we set the DataConext with the ViewModel for the View:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        DataContext = new MainViewModel();
        InitializeComponent();
    }
}

Lastly, we can now wire up the view:

<StackPanel Orientation="Horizontal">
    <StackPanel.Resources>
        <Style TargetType="Button">
            <Setter Property="Padding" Value="20 10" />
            <Setter Property="Margin" Value="5 10"/>
        </Style>
    </StackPanel.Resources>

    <Button Content="Button 1"
            Command="{Binding ButtonClickCommand}"
            CommandParameter="Button 1" />
    <Button Content="Button 2"
            Command="{Binding ButtonClickCommand}"
            CommandParameter="Button 2" />
    <Button Content="Button 3"
            Command="{Binding ButtonClickCommand}"
            CommandParameter="Button 3" />
</StackPanel>

<TabControl Grid.Row="1" Height="100"
            ItemsSource="{Binding Tabs}">
</TabControl>

When you run the code, each Button click will notify the ViewModel via the Command and a new Tab will be added to the Tabs collection. The Tabs colllection will notify the View via the binding and the Tabs control will add the new Tab.

This is the bare minimum to getting it working. The rest is up to you.

Hope this answers your question.

<TargetFramework>net6.0-windows</TargetFramework>

ButtonClickCommand = new ButtonClickCommand(s => OnClicked(s));