Assinging a value to a drop-down box

I have a drop-down box which is populated at form load by a query from a
look-up table in the database (the query captures a number and the
corresponding text, and only the text is shown). How can I programatically
(from a button click) tell the combo box to "now pick this selection from
the list", based on the text value? So there are 15 distinct (and only 15)
possible values - say flavors. If there is a database value for that field
it will pick it up for the combo box. But if there isn't, I want to click a
button that says "assign this one the value of Chocolate" (or whatever
flavor is appropriate based on other criteria).