How to assign VolumeParameter for namespace UnityEngine.Rendering in script

Graphics Render Pipelines
,
1

line 17 is not working. I can’t simply assign it a float or assign it in inspector. Anyone know how?

using UnityEngine;
using UnityEngine.Rendering.HighDefinition;
using UnityEngine.Rendering;

public class SkyController : MonoBehaviour
{

    public HDRISky sky;

 
    public VolumeParameter vol;
    void Start()
    {
        sky = GetComponent<HDRISky>();

        vol = 4f;
    }

    public void ChangeSky()
    {
        sky.desiredLuxValue.SetValue(vol);


    }
}
2 
m_Volume.sharedProfile.TryGet<HDRISky>(out var Temp0);
        //Debug.Log(Temp0.exposure);
        //Temp0.multiplier = new MinFloatParameter(expmax, 1,true);
        Temp0.desiredLuxValue = new FloatParameter(expmax, true);

it can be done but it cannot refresh after the code runonce