how to raycast against a mesh instead of the object in unity

So I am trying to make random map generation for a game, and right now I am trying to use a raycast to get the height of the spot where a prefab will be generated, I am using a terrain and generating the mesh overtop, the mesh has a mesh collider on it. Right now, the only thing that happens is the raycast hits the terrain itself, how can I make it hit the mesh instead. here is my random position code:

    public void InstantiateRandomPosition(int Amount, float AddedHeight)
{
    var i = 0;
    float terrainHeight = 0f;
    RayCastHit hit;
    float randomPosX, randomPosY, randomPosZ;
    Vector3 randomPos = Vector3.zero;

    do
    {
        i++;
        randomPosX = Random.Range(TerrainLeft, TerrainRight);
        randomPosZ = Random.Range(TerrainBottom, TerrainTop);

        if (Physics.Raycast(new Vector3(randomPosX, 99999f, randomPosZ), Vector3.down, out hit, Mathf.Infinity, TerrainLayer))
        {
            terrainHeight = hit.point.y;
        }

        randomPosY = terrainHeight + AddedHeight;

        randomPos = new Vector3(randomPosX, randomPosY, randomPosZ);

        Instantiate(Test, randomPos, Quaternion.identity);

    } while (i < Amount);

}

can somebody help?