I don’t really know what you mean by “now”.
Yes, you can give velocity commands to Ardupilot, but only if Ardupilot can estimate it’s own velocity. In your flight logs, do you see velocity estimates? XKF1.VE and XKF1.VN (Onboard Message Log Messages — Copter documentation)
Another thing I found when reading posts exactly like yours (Navigation with Optical Flow Only - #3 by moobsen):
On mission Planner, you can set the ekf home position using the right button of the mouse.\
I’m not exactly sure if this is still necessary, but the EKF must set a home position before flying in Guided. This happens automatically when flying outside with a GPS, but maybe not if flying with optical flow.
Another post: The copter does not takeoff in Guided mode and EKF_GPS_TYPE = 3 (optical flow) - #3 by dollop
But to reiterate, optical flow does not provide a velocity estimate on its own. You need a rangefinder with your optical flow sensor to estimate (and control) velocity.