New Zwifter. How to calibrate for erg mode with power meter and kickr

I understand how to calibrate(spindown) my kickr snap, but for regular zwift rides, I just link my stage power meter to zwift
My question is for zwift workouts, when I try to spindown, the wahoo app stops calibration, IF I am off zwift it calibrates fine.
What do you do?
Thanks in advance