How can you use ffmpeg's afftdn -sn flag to sample noise from a reference noise file?
Commands are instructions sent from an interface external to the filter, usually an external program using a ZMQ protocol. There is a filter that can execute commands which is what I'll use here. 

Base syntax is

ffmpeg -i in -af "asendcmd=c='0.0 afftdn@n sn start; 1.0 afftdn@n sn stop',afftdn@n" noiseout.wav


To use another source for noise profile, you'll have to append it, measure noise, and then remove it.

ffmpeg -i in -i profile -filter_complex "[1][0]concat=n=2:v=0:a=1,asendcmd=c='0.0 afftdn@n sn start; 1.0 afftdn@n sn stop',afftdn@n,atrim=1,asetpts=PTS-STARTPTS" noiseout.wav


Above, I assume the profile is 1 second long, which is removed with atrim after denoising.