I see that's a multibox you've got there,am I right?
The setting in the multibox for the search style is not in an obvious place. Go to the properties, then the presentation tab. Highlight a field expression by clickingon it, then click the"advanced" button from the options on the right. The default search mode you are looking for is the wildcard search.
Does this work for you?
If your input box options are limited you could "fake it" with a list box that links to a stand-alone field comprising the drop down items you wish to use. Then you could link your variable to the selection in that field. If you set the list box to "always one selected value" that will prevent users fromselecting more than one item.