Create an unbounded form.
Insert an unbound combo box and use a customer list query as its row source
In your parameter queryreplace the:
Like [Insert Customer] or whatever you are using to gererate the parameter box
to:
Like Forms!FormName!ComboName
NB:
Formname is the name of the unbounded form
comboname is the name of the combox on the unbound form
Then create a button on the unbounded form and set its onclick event to:
docmd.openquery "QueryName"
Alternatively you can run the openquery on the onchange or afterupdate event of the combobox