Attach behavior in code behind

XamComboEditorSelectedItemsBehavior behavior = new XamComboEditorSelectedItemsBehavior();
behavior.SetBinding(XamComboEditorSelectedItemsBehavior.SelectedItemsProperty, new Binding() 
    { 
        ElementName = "_uc", 
        Path = new PropertyPath("SelectedItems"), 
        Mode = BindingMode.TwoWay 
    });
Interaction.GetBehaviors(yourElementName).Add(behavior)