The WatermarkComboBox derives from the ComboBox and adds a watermark. So the filtering “Starts With” is the same as the one in the standard comboBox.
If you want to use the WatermarkComboBox and use a “Contains” filter instead of a “Starts With” filter, you will have to derive from ComboBox (or WatermarkComboBox) and add the necessary to filter the way you want. Here are people trying to do so :
Please note that the toolkit also offers a TokenizedTextBox with filtering as you type. You can test it in the sample “Input Fields/Text/Tokenized TextBox” of the LiveExplorer of the Toolkit available here : http://forums.xceed.com/xceed-toolkit-plus-for-wpf/.