To enable holds on an on-order title, you need to create a dummy copy record with a barcode. You can use a real barcode or a dummy one, e.g. NLCLIBRARYCODE00001. You can change the copy’s status from In Process to On Order, which is OPAC visible and holdable.
When the item comes in you will need to update the copy record. For example, you will need to update copy status, and if you used a dummy barcode, you will need to edit the copy barcode.
For further information see the Sitka Cataloguing Policy – http://docs.libraries.coop/policy/_cataloguing_procedures.html#_working_with_on_order_item_records