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/.