Passing a custom parameter to a standard report form

I am trying to add a filter to the Scheduled Shipment report that was requested as:

a drop down list with two options (aside from the obvious blank), where if they choose 1 option only jobs that have the job code (and optional field from the JobHead) with that code will print, if they choose 2nd option, only jobs that that that job code will print, otherwise all will print. 

Any idea how to pass their choice from a combo box to the report parameters?