How can I display what the camera sees on the phone with UI builder?

Hi everyone, I am very new to Unity.

I am currently working on a project that rebuilds a camera app with unity. Due to some reasons (to integrate with teammates), I am using the UI Builder to create the UI of a camera, which contains a Button to shoot and a Visual Element area to display what the camera captures.

My script on the camera supposed to be like this:

 public class PhoneCamera : MonoBehaviour
 {
     private bool camAvailable;
     private WebCamTexture frontCam;
     private Texture defaultBackrgound;
 
     public VisualElement displayArea; //Which should links to the visual element created in the UI Builder
     public AspectRatioFitter fit;
 
     void Start()
     {
         defaultBackrgound = displayArea.texture;
         WebCamDevice[] devices = WebCamTexture.devices;
         ...
         frontCam = new WebCamTexture(devices[i].name, Screen.width, Screen.height);
         ...
         frontCam.Play();
         displayArea.texture = frontCam;
         camAvailable = true;
      }
 }

Where I got errors saying that the visual element has no texture.

So my question is:

How to access to and change the background texture of the visual element so I can display what the camera sees on it?

Thanks in advance!