Use the Layout Editor.

Select the combo box element
Use the Property Edtor
Click on the tab for other
Enter the Initial Value into the fleld for Initial Value, 4th field down.
Click on the arrow to the right to assign that Initial Value to the selected Combo Box.
Make sure the spelling matches one of the choices available.
You can press F6 to expand the field for Initial Value if necessary.  When done, press F5 again to Save and Close.
Remember to click the arrow on the right.