Need to copy BKTXT field value to XREF3 field - for residual documents

HI

We setup the Lockbox automatch key field as XREF3 , but our standard mapping for invoices from Interfaces is to post the invoice number to BKTXT field .

To have the residual document identified by Lockbox file, now the requirement is to copy the BKTXT field to XREF3 as well .

I am not sure if the substitution works, is there a better way to setup this logic.

Thanks and Regards