Using form for a query

I'm using a form to set parameters for a query, and eventually a report. I
want to query for all shipments going to a specific city, selected from a
combo box. This works fine if I ask the query to look back at the form for
the destination criteria.

However, I want users to be able to select a wildcard-type selection that
would have the query run for all destinations. I've tried using a blank and
* to no avail, so I'm guessing I will need to build a SELECT string to pass
along as part of the DoCmd, but I don't know how to do this.

Can someone please help?