If you are using the ReportService, currently, there's no way to filter on
custom field values in the API's
ReportService. You can include the custom field value in your report by setting its ID in your ReportQuery:
You could then perform post-processing on your end to filter out the desired value.
Another workaround would be to filter your report query on Order IDs. You could use the
OrderService to determine the IDs of the Orders with your desired custom field value and then use those IDs in your report query's filter