According to the manual, FR255 is able to "automatically detect your functional threshold power (FTP) during an activity" : https://www8.garmin.com/manuals/webhelp/GUID-676967A0-1B23-4384-9BC9-76F3D643F1C8/EN-US/GUID-0EDC68E3-D886-48D2-B624-AA634739B421.html
But my watch does not have an option to automatically detect FTP in User Profile > Heart Rate & Power Zones > Auto Detection.
As you can see it only has options for max heart rate and lactate threshold.
and is missing FTP :
Ping Garmin-Kevin since you locked the other discussion even though your answer is incomplete and only mentions FTP test.