Here's what is in the 2019 edition of what used to be called the Airman's Information Manual:
Distress and Urgency Communications
A pilot who encounters a distress or urgency condition can obtain assistance simply by contacting the air traffic facility or other agency in whose area of responsibility the aircraft is operating, stating the nature of the difficulty, pilot's intentions and assistance desired. Distress and urgency communications procedures are prescribed by the International Civil Aviation Organization (ICAO), however, and have decided advantages over the informal procedure described above.
https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap6_section_3.html