use NOTE endpoint
use POST method
use this or similar body for the call:
{"auditType":"NO","noteObjCode":"[objCode where you are posting note]","objID":"[objID where you post note]","taskID":"[provide taskID when posting note on a task]","opTaskID":"[provide issueID when posting note on a issue]","parentNoteID":"[add when continue a conversation Thread]","noteText":"[input note text here]","tags":"[add tags if you want to notify someone about the update]"}