How to jump to combo box selection when keying in multiple characters (not just first character)

I have a PDF combo box with a list of over 300 different locations, beginning with location numbers. I have a script in Custom Calculation to allow the combo box to be editable (see below). Still, when you begin typing in a location number, it only searches by a single digit matching the first digit of a selection. For instance, if I want to find location 803, typing in "803" only brings up the first instance starting with a "3" since it was the last digit entered. If you key in "3" multiple times, it jumps through the selections starting with "3." I would like to script it to jump to whatever I fully key in, such as "803." I would especially like it to jump to the location name if that is keyed in, though the listing does not start with names. Any suggestions or sample scripts would be helpful.

this.getField("ddlUnitType.1").editable = true;