Excerpt from Brackeys video tutorial.

PerlinNoise.cs

using UnityEngine;

[RequireComponent(typeof(MeshFilter))]
public class PerlinNoise : MonoBehaviour
{
    [SerializeField] int width = 256;
    [SerializeField] int height = 256;

    [SerializeField] float scale = 20f;

    [SerializeField] float offsetX = 100f;
    [SerializeField] float offsetY = 100f;

    Renderer meshRenderer;

    #region Unity Method
    void Start()
    {
        offsetX = Random.Range(0f, 99999f);
        offsetY = Random.Range(0f, 99999f);
        meshRenderer = GetComponent<Renderer>();
    }

    void Update()
    {
        meshRenderer.material.mainTexture = GenerateTexture();
    }
    #endregion

    #region Custom Method
    Texture2D GenerateTexture()
    {
        Texture2D texture = new Texture2D(width, height);

        for (int x = 0; x < width; x++)
        {
            for (int y = 0; y < height; y++)
            {
                texture.SetPixel(x, y, CalculateColor(x, y));
            }
        }

        texture.Apply();
        return texture;
    }

    Color CalculateColor(int x, int y)
    {
        float xCoord = (float)x / width * scale + offsetX;
        float yCoord = (float)y / height * scale + offsetY;
        
        float sample = Mathf.PerlinNoise(xCoord, yCoord);

        return new Color(sample, sample, sample);
    }
    #endregion
}