If you're using trainerroad and a smart trainer it should set it automatically (check this is the case with your trainer, maybe the trainer itself needs to be set manually to ERG?).
You shouldn't be using ERG mode for normal FTP tests but for Ramp Tests, TR will enable it.
No gear changes, no position changes, no finishing sprints, just get into race position, ride as long as you can at basically the same cadence and stop when you can't keep going.
If you're using trainerroad and a smart trainer it should set it automatically (check this is the case with your trainer, maybe the trainer itself needs to be set manually to ERG?).
You shouldn't be using ERG mode for normal FTP tests but for Ramp Tests, TR will enable it.
https://support.trainerroad.com/hc/en-us/articles/201869764-Erg-Mode-Explained
https://www.smartbiketrainers.com/trainerroad-makes-ftp-testing-suck-less-with-a-new-ramp-test-5266