Converting unity OnRenderImage() to URP

Question

I am making a game with a pixelation shader. I downloaded this shader from the web, and it works with the default render pipeline. I want to use the URP for other purposes. When using the URP, the pixelation shader did not work. I narrowed the problem down a little bit. I found out that OnRenderImage() does not get called using the URP, but does get called with the default RP. I think there is a way to do this because of this thread. I don't understand what they are talking about because I am very new to shaders. I searched further and found this in the unity manual. However, I do not understand it. Can someone walk me through it, and convert this script to be compatible with the URP?

Here is the script:

using UnityEngine;
using System.Collections;
[ExecuteInEditMode]
[RequireComponent(typeof (Camera))]
[AddComponentMenu("Image Effects/Pixelate")]
public class Pixelate:MonoBehaviour{
    public Shader shader;
    int _pixelSizeX=1;
    int _pixelSizeY=1;
    Material _material; //pixelation material/shader
    [Range(1,20)]
    public int pixelSizeX=1;
    [Range(1,20)]
    public int pixelSizeY=1;
    public bool lockXY=true;
    void OnRenderImage(RenderTexture source, RenderTexture destination){
        if(_material==null) _material=new Material(shader);
        _material.SetInt("_PixelateX",pixelSizeX);
        _material.SetInt("_PixelateY",pixelSizeY);
        Graphics.Blit(source,destination,_material);
    }
    void OnDisable(){
        DestroyImmediate(_material);
    }

    void Update(){
        if(pixelSizeX!=_pixelSizeX){
            _pixelSizeX=pixelSizeX;
            if(lockXY) _pixelSizeY=pixelSizeY=_pixelSizeX;
        }
        if(pixelSizeY!=_pixelSizeY){
            _pixelSizeY=pixelSizeY;
            if(lockXY) _pixelSizeX=pixelSizeX=_pixelSizeY;
        }
    }
}