public class SomeClass : INotifyPropertyChanged
{
    public SomeClass() {}

    public IList<string> Categories
    {
        get
        {
            return _categories;
        }
        set
        {
            _categories = value;
            OnPropertyChanged(nameof(Categories));
        }
    }

    public IList<string> Forms
    {
        get
        {
            return _forms;
        }
        set
        {
            _forms = value;
            OnPropertyChanged(nameof(Forms));
        }
    }

    public void OnPropertyChanged(string propertyName)
    {
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

<Window.Resources>
    <local:SomeClass x:key="MyClass"/>
</Window.Resources>

<ToolBarPanel x:Name="myToolStripName" DataContext="{StaticResource MyClass}">
    <ToolBarTray>
        <ToolBar Height="25px">
            <ComboBox x:Name="cboCategories"
                      ItemsSource="{Binding Path=Categories}"
                      SelectionChanged="CboCategories_SelectionChanged" />
            <ComboBox x:Name="cboForms"
                      ItemsSource="{Binding Path=Forms}" />
        </ToolBar>
    </ToolBarTray>
</ToolBarPanel>

private void CboCategories_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
{
    string categoryName = e.AddedItems[0].ToString();
    if (!string.IsNullOrEmpty(categoryName))
    {
        [problem: MyClass/SomeClass].GetFormNames(categoryName);
    }
}