MyCanvasScaler.cs

using UnityEngine;
using UnityEngine.UI;

[RequireComponent(typeof(Canvas))]
[RequireComponent(typeof(CanvasScaler))]
public class MyCanvasScaler : MonoBehaviour
{
    [SerializeField] GameObject canvasContent;
    [SerializeField] bool keepProportions = true;
    Vector2 referenceResolution;


    void OnValidate()
    {
        if (canvasContent != null)
        {
            // sets the original CanvasScaler settings
            var canvasScaler = GetComponent<CanvasScaler>();
            canvasScaler.uiScaleMode = CanvasScaler.ScaleMode.ScaleWithScreenSize;
            this.referenceResolution = canvasContent.transform.GetComponent<RectTransform>().sizeDelta;
            canvasScaler.referenceResolution = this.referenceResolution;
            canvasScaler.screenMatchMode = CanvasScaler.ScreenMatchMode.Expand;
        }
    }

    void Update()
    {
        if (canvasContent != null && !keepProportions)
        {
            // if requested scales the canvas content, creating a full-screen distorted version
            float currentWidth = GetComponent<RectTransform>().sizeDelta.x;
            float currentHeight = GetComponent<RectTransform>().sizeDelta.y;

            float scaleX = currentWidth / referenceResolution.x;
            float scaleY = currentHeight / referenceResolution.y;

            canvasContent.transform.GetComponent<RectTransform>().localScale = new Vector2(scaleX, scaleY);

        }

    }
}