There is an example
here[
^]
It could do with some improvement... e.g. capture the current listindex in the keydown event (if it is not -1) so you can get the "next" match (roll around if not found)
There are other examples using other methods - follow the suggestion from Wes Aday for a google search.
Or change the style of the combobox to 2 - Dropdown list and the user just needs to hit the first character of the option they need ... multiple hits will cycle through matching entries