Publish to topic vel_cmd of plugin Diff drive

I have added the model Pioneer 2DX to an empty world. This model comes with the plugin [DiffDrive](https://bitbucket.org/osrf/gazebo/src/9e998023acbb0417ca6214047d0a102c9984b927/plugins/DiffDrivePlugin.hh?at=default).left_wheel_hingeright_wheel_hinge5 Running `gz topic -l` I get the topic: /gazebo/default/pioneer2dxCamera/vel_cmd How do I control the robot without using ROS? I'm getting crazy figuring out how to publish to topic vel_cmd to control this robot.