"error_description": "missing required field \"grant_type\""

{
"body": {
"grant_type": "",
"refresh_token": "<token>"
},
"contentType": "application/json",
"followRedirect": false,
"headers": [
{
"key": "Authorization",
"value": "Basic <key>"
}
],
"method": "POST",
"timeoutSeconds": 5,
"url": "https://api.dropbox.com/oauth2/token"
}

​

Output:

{
  "error": {
    "canCatch": true,
    "canRetry": true,
    "cause": "invalid response status code",
    "description": "The HTTP request failed",
    "error": "Flow.Step.HTTPRequest.Failed",
    "metadata": {
      "body": {
        "error": "unsupported_grant_type",
        "error_description": "missing required field \"grant_type\""
      },
      "headers": {
        "Accept-Encoding": [
          "identity,gzip"
        ],
        "Content-Length": [
          "97"
        ],
        "Content-Security-Policy": [
          "sandbox allow-forms allow-scripts"
        ],
        "Content-Type": [
          "application/json"
        ],
        "Date": [
          "Tue, 07 Nov 2023 11:58:41 GMT"
        ],
        "Server": [
          "envoy"
        ],
        "X-Dropbox-Request-Id": [
          "6857e5f3655547459b471405aef54adc"
        ],
        "X-Dropbox-Response-Origin": [
          "far_remote"
        ]
      },
      "statusCode": 400,
      "statusText": "Bad Request"
    }
  }
}