Sending velocity command in SITL

I am using SITL to simulate the flight of my fixed wing aircraft. I have managed to load and fly a mission. Now I would like to send velocity commands directly. I have tried to do this with the Mavproxy velocity command and over Mavros on the topic mavros/setpoint_velocity/cmd_vel, but nothing works. I have tried it in many different flight modes, such as auto, guided, fbwb and manual, but nothing happens when I send the command.

What is the correct way to send velocity commands?

Thanks a lot!