You need to include an item "Test input" and set IsSelected="True" e.g.
<ComboBox IsEditable="False" >
<ComboBoxItem IsSelected="True">
Test Input
</ComboBoxItem>
</ComboBox>
[Edit - further info after OP clarification of requirement]
There are various options, rather than re-inventing the wheel I suggest reading the suggestions on the following links and choose the method that best suits your application
SO Post 1[
^]
SO post 2[
^]