boto3 generate_presigned_post failed to set custom meta tag

Trying to generate pre-signed s3 upload url with below code:

response = s3_client.generate_presigned_post(
  SOURCE_S3_BUCKET,
  file_name,
  Fields={"x-amz-meta-test-tag": "test-tag"},
  Conditions=None,
  ExpiresIn=3600,
)

After I got the response, I tried setting the x-amz-meta-test-tag in both form values and headers. My HTML form

<form action="URL_VALUE" method="post" enctype="multipart/form-data">
      <input type="hidden" name="key" value="VALUE" />
      <input type="hidden" name="AWSAccessKeyId" value="VALUE" />
      <input type="hidden" name="policy" value="VALUE" />
      <input type="hidden" name="signature" value="VALUE" />
      <input type="hidden" name="x-amz-meta-test-tag" value="test-tag" />
    File:
      <input type="file"   name="file" />
      <input type="submit" name="submit" value="Upload to Amazon S3" />
    </form>

However, none of them seem to work. The file is uploaded to s3 successfully. I just couldn't set the meta tag