ProceduralGrass.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ProceduralGrass : MonoBehaviour
{
    private Terrain terrainToPopulate;
    public int grassDensity;
    public int patchDetail;
    [ContextMenu("Grass")]
    void Start()
    {
        terrainToPopulate = transform.gameObject.GetComponent<Terrain>();
        terrainToPopulate.terrainData.SetDetailResolution(grassDensity, patchDetail);

        int[,] newMap = new int[grassDensity, grassDensity];

        for (int i = 0; i < grassDensity; i++)
        {
            for (int j = 0; j < grassDensity; j++)
            {
                newMap[i, j] = 6;
            }
        }
        terrainToPopulate.terrainData.SetDetailLayer(0, 0, 0, newMap);
    }
}