Create the combo box on an unbound form and reference it as the parameter in
the query, e.g.
SELECT *
FROM Contacts
WHERE LastName = Forms!YourForm!YorComboBox;
Then open the query, or better a form or report based on the query with a
button on a form.
In the above example the RowSource for the combo box would be:
SELECT DISTINCT
LastName
FROM Contacts
ORDER BY LastName;
Ken Sheridan
Stafford, England