Enable barcode/accession number search in OPAC

Add following query in
Koha> Administration › System preferences › OPACUserJS

$("select[name='idx']").append("<option value='bc'>Accession No.</option>");

Save.
Refresh OPAC page and see the change.

Information courtesy:
L2C2 Technologies
http://blog.l2c2.co.in/index.php/2016/09/10/quick-tip-add-barcode-lookup-to-your-opacs-search-index-selection-downdown/