Can the newest iMac be used as a secondary monitor for my 2018 MacBook Pro? If so, what adapter do I need?

If you are referring to something like using the new iMac in Target Display Mode, then AFAIK, Apple has not designed any of their iMacs to support this feature since the Late 2014 models. I would suspect that the recently announced M1 models to either.

Ref: Use your iMac as a display with Target Display Mode - Apple Support

TDM has been dead since 2015, so the short answer is no. If your MBP needs an external display, please buy one. Macs are computers they are not displays for other devices.