WatermarkComboBox auto filtering / searching

WatermarkComboBox auto filtering / searching

I am using the WatermarkComboBox and when I have the item source hooked up so I can type in the combo box and it will filter the list as I type.

The problem I am having is it seems to be doing the filtering on the start of the string and I would like to do it on if the string contains what I’m typing.

For example:
With a list of:

When I start typing “Ke” it does not match to anything, and I would like it to match “DOMAIN\Kevin”

My usage:

                        <xctk:WatermarkComboBox Watermark="DOMAIN\USERNAME" IsEditable="True"
                        VerticalAlignment="Center" HorizontalAlignment="Center" Width="150"
                        Text="{Binding UserName, UpdateSourceTrigger=PropertyChanged}"
                        SelectedItem="{Binding UserName}"
                        ItemsSource="{Binding UserList }"/>

Thank you for any help on this matter.