How do I create a comment with reddit's api using fetch?

async function commentOnRedditPost(postId, commentText, accessToken) {
  const url = `https://oauth.reddit.com/api/comment`;
  // const params = new URLSearchParams();

  console.log("access_token:", accessToken, postId, commentText);

  // params.append("parent", postId); // 'thing_id' is used instead of 'parent'
  // params.append("text", commentText);

  const response = await fetch(url, {
    method: "POST",
    headers: {
      Authorization: `Bearer ${accessToken}`,
      "Content-Type": "application/json",
      Accept: "text/html",
      "User-Agent": "profullstack comment bot v1.0",
      "Sec-Fetch-Dest": "document",
      "Sec-Fetch-Mode": "navigate",
      "Sec-Fetch-Site": "cross-site",
    },
    mode: "cors",
    body: JSON.stringify({
      api_type: "json",
      thing_id: postId,
      text: commentText,
    }),
  });

  if (!response.ok) {
    console.error(await response.text());
  }

  return response.json();
}

I get a 403 and a body parsing error:

{"message": "Forbidden", "error": 403}
Failed to comment on Reddit post 1cxbmk6: TypeError: Body is unusable

Needed to use thing_id: post.name, instead of postId. It includes the "type" ie: t3_$id