Looks like you will need to modify the server's jar file for that. See the quote below. The jar file can be opened using 7zip. I'm not sure the exact path inside the jar file either.
In this case, Forge will already be in the server JAR. You won't want to erase all of META-INF or the server won't launch, just deleting FORGE.DSA is enough according to DAOWAce's post below. Then just copy the class file you downloaded into the appropriate place in the JAR.