Velocity Control uses the PID controller to run the motor at a set speed in RPM. An FF gain, as detailed in F Parameter, is your friend in finding consistency when tuning this mode.
It is called in the same way as Position Control:
API Docs: setReference
API Reference: SetReference
Velocity Control mode will turn your motor continuously,. Be sure your mechanism does not have any hard limits for rotation.
Velocity Loop constants are often of a very low magnitude, so if your mechanism isn't behaving as expected, try decreasing your gains.