<ScrollViewer >
    <StackPanel>
        <DockPanel LastChildFill="False">
            <TextBox DockPanel.Dock="Left"/>
            <Button DockPanel.Dock="Right"
                    Content="+"/>
        </DockPanel>
    </StackPanel>
</ScrollViewer>

// INotifyPropertyChanged if you need, as well as full-properties with notification
public class Item
{
    public string Text {get; set;}
}

public ObservableCollection<Item> Items { get; } = new ObservableCollection<Item>();

void OnCommandAdd() => Items.Add(new Item());

<ListBox ItemsSource="{Binding Items}">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <TextBox Text="{Binding Text}" />
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>