how to add a subject line to iMessage

I noticed you tagged your question for iMac. There is no subject line option in Messages for Mac OS.

It can be done in Messages on an iPhone, which is odd because there is so little space on a phone screen anyway would you want to consume some of it with an additional line?

Need to figure out how to add the subject line back in MacOS, looks like setting has been moved in MacOS 15 now.