To turn on barcode/QR code scanning for the Record Finder, you need to go to:
Your linked record field's settings
Record Finder Options section
Record finder mode should be set to List
Turn on Allow users to scan barcodes & QR codes in finder
Optional: Turn off Automatically start camera to prevent the camera from opening as soon as the record finder is opened.
Optional: Turn off Auto submit after scanning to prevent the form from linking the record as soon as a code has been scanned successfully and close the record finder.