How to assign VolumeParameter for namespace UnityEngine.Rendering in script

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

Code (CSharp):

1.  
2. using UnityEngine;
3. using UnityEngine.Rendering.HighDefinition;
4. using UnityEngine.Rendering;
5.  
6. public class SkyController : MonoBehaviour
7. {
8.  
9.     public HDRISky sky;
10.  
11.  
12.     public VolumeParameter vol;
13.     void Start()
14.     {
15.         sky = GetComponent<HDRISky>();
16.  
17.         vol = 4f;
18.     }
19.  
20.     public void ChangeSky()
21.     {
22.         sky.desiredLuxValue.SetValue(vol);
23.  
24.  
25.     }
26. }
27.  

 

Code (CSharp):

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

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