Adding a subject line to a message makes it a lot more scannable for everyone else. To do that, first you need to expand your compose box by clicking format icon. From there, just type the subject line you want in the first line of your expanded compose box.
An error occurred while saving the commentAnonymous commented
No, you don't have that. The suggested subject line thing does not work. The Chat in the left of the Teams UI shows only the last chat message, and the subject is always on the first line. So, the subject is lost to view almost immediately and the list of chats on the left are wholly without context. If you're going to tell me to search the chats for various phrases, again, that is a miss because people remember what a chat is about not what was specifically said.