Getting 400 Error for some unknown reason

After receiving my response from https://www.bungie.net/en/OAuth/Authorize with my code I am attempting to get my token using the following Axios POST request:

const buff = new Buffer.from("<client-id>:<client-secret>", "utf-8");

    await axios.post(`https://www.bungie.net/Platform/App/OAuth/Token/?grant_type=authorization_code&client_id=<client-id>&code=${req.originalUrl.split("code=")[1]}`, {}, 
      {
        headers: {
            'Content-Type': 'application/x-www-form-urlencoded',
            'Authorization': 'Basic ' + buff.toString('base64'),
            'X-API-Key': '<my-api-key>',
        },
      }).then((resp)=>{
        return res.status(200).json({
          message: resp,
        });
      })
      .catch(err => {
        return res.status(500).json({
          message: err,
        });
    });

I've looked through almost all of the posts similar to the issue I am having and tried most of the solutions, but to no avail. Here is the specific error code I am getting back for reference:

{
  "message": {
    "message": "Request failed with status code 400",
    "name": "AxiosError",
    "stack": "AxiosError: Request failed with status code 400\n    at settle (/var/task/node_modules/axios/dist/node/axios.cjs:1855:12)\n    at IncomingMessage.handleStreamEnd (/var/task/node_modules/axios/dist/node/axios.cjs:2712:11)\n    at IncomingMessage.emit (events.js:412:35)\n    at endReadableNT (internal/streams/readable.js:1333:12)\n    at processTicksAndRejections (internal/process/task_queues.js:82:21)",
    "config": {
      "transitional": {
        "silentJSONParsing": true,
        "forcedJSONParsing": true,
        "clarifyTimeoutError": false
      },
      "adapter": [
        "xhr",
        "http"
      ],
      "transformRequest": [
        null
      ],
      "transformResponse": [
        null
      ],
      "timeout": 0,
      "xsrfCookieName": "XSRF-TOKEN",
      "xsrfHeaderName": "X-XSRF-TOKEN",
      "maxContentLength": -1,
      "maxBodyLength": -1,
      "env": {
        "Blob": null
      },
      "headers": {
        "Accept": "application/json, text/plain, */*",
        "Content-Type": "application/x-www-form-urlencoded",
        "Authorization": "Basic <base64-encoded-string>",
        "X-API-Key": "<my-api-key>",
        "User-Agent": "axios/1.2.1",
        "Accept-Encoding": "gzip, compress, deflate, br"
      },
      "method": "post",
      "url": "https://www.bungie.net/Platform/App/OAuth/Token/?grant_type=authorization_code&client_id=<client-id>&code=2ad2d3e2ffad30208d92d706c32605ab",
      "data": ""
    },
    "code": "ERR_BAD_REQUEST",
    "status": 400
  }
}

Some help would be much appreciated! Thank you.

Bungie-net / api

Getting 400 Error for some unknown reason #1752