load a tileset PNG from the streamingAssets folder, and create a Unity Tilemap and all necessary objects with a C# script

TilemapTest.cs

using UnityEngine;
using UnityEngine.Tilemaps;

// Load a tileset PNG from the streamingAssets folder, and create a
// Unity Tilemap and all necessary objects with a C# script.
// Attach this script to an empty GameObject.

// Copyright 2020 by Frank Buss. Can be used for free in any project.

public class TilemapTest : MonoBehaviour
{
    public static Texture2D LoadTexture(string filename)
    {
        // load an image file rom the streamAssets folder
        byte[] pngBytes = System.IO.File.ReadAllBytes(Application.streamingAssetsPath + "/" + filename);

        // create a Texture2D with it
        Texture texture = new Texture();
        Texture2D tex2d = new Texture2D(2, 2);
        tex2d.filterMode = FilterMode.Point;
        tex2d.LoadImage(pngBytes);
        return tex2d;
    }

    private Tile CreateTile(Texture2D tileset, int x0, int y0, int width, int height)
    {
        // create a sprite from a part of the texture
        Sprite tileSprite = Sprite.Create(tileset, new Rect(x0, y0, width, height), new Vector2(0, 0), width);

        // create a tile with the sprite
        Tile tile = ScriptableObject.CreateInstance(typeof(Tile)) as Tile;
        tile.sprite = tileSprite;

        return tile;
    }

    public void Awake()
    {
        // create 2 tiles from a tileset with 32x32 pixel tiles, which have one pixel spacing
        Texture2D tileset = LoadTexture("tmw_desert_spacing.png");
        Tile tileA = CreateTile(tileset, 1, 1, 32, 32);
        Tile tileB = CreateTile(tileset, 34, 1, 32, 32);

        // create the Unity tilemap gameobjects
        GameObject gridGameObject = new GameObject("Tilemap Grid");
        var grid = gridGameObject.AddComponent<Grid>();
        GameObject tilemapGameObject = new GameObject("Tilemap");
        tilemapGameObject.transform.parent = gridGameObject.transform;
        var tm = tilemapGameObject.AddComponent<Tilemap>();
        tilemapGameObject.AddComponent<TilemapRenderer>();

        // create a checkerboard with the 2 tiles
        Vector2Int size = new Vector2Int(8, 8);
        Vector3Int[] positions = new Vector3Int[size.x * size.y];
        TileBase[] tileArray = new TileBase[positions.Length];
        int index = 0;
        for (int y = 0; y < size.y; y++)
        {
            for (int x = 0; x < size.x; x++)
            {
                positions[index] = new Vector3Int(x, y, 0);
                tileArray[index] = (x + y) % 2 == 0 ? tileA : tileB;
                index++;
            }
        }
        tm.SetTiles(positions, tileArray);
    }
}