I define a local printer on a desktop system, and then try to access it remotely via s-c-p on a laptop.  I pick "Network Printer", then "Find Network Printer", and fill in the desktop hostname.  s-c-p correctly finds the desktop printer, and creates the URL ipp://desktop:631/printers/(printername).

When I click Forward, I get a password prompt with the User filled in as "root" and Domain filled in as "MYGROUP" with no password.  I've tried the desktop root password and also my own ID and password (both these passwords are identical with their counterparts on the laptop).  Authentication always fails.

Then I realized that the presence of MYGROUP indicates that authentication is being done against Samba, and neither system has any Samba IDs defined.

If I add the remote printer from CUPS using the same ipp URL as above, I get no authentication prompt (other than the local CUPS Administration one) and the remote printer is created successfully with the ippeve.ppd (IPP Everywhere) PPD.