How to confige an official plugin in model file?

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Hi everyone! Now I was confuse about how to setting the parameters of some official plugin in model file(such as urdf,xacro,sdf...)

As the official tutorial has said,

Modify the end of the Load function to read the <velocity> using the sdf::ElementPtr parameter.

// Default to zero velocity
double velocity = 0;

// Check that the velocity element exists, then read the value
if (_sdf->HasElement("velocity"))
  velocity = _sdf->Get<double>("velocity");

// Set the joint's target velocity. This target velocity is just
// for demonstration purposes.
this->model->GetJointController()->SetVelocityTarget(
    this->joint->GetScopedName(), velocity);

Actually, I can only see the members of the class on the official documentation.

So, I was wondering how to get the parameter which is needed to confige the plugin? Like

<gazebo reference="your_link_name">
  <plugin name="your_link_laser_controller" filename="libgazebo_ros_laser.so">
    ... plugin parameters ...
  </plugin>

Thanks a lot!