How to get uppy/examples/aws-presigned-url to work?

I’m trying to run npm run example aws-presigned-url, but I don’t the faintest idea of what to write for COMPANION_AWS_ENDPOINT

I’m realising the problem is not there, leaving it empty and trying to debug the PHP (s3-sign.php):

$url = (string) $request->getUri();
file_put_contents($file, "+++$url\n", FILE_APPEND);

now I see the error when loading the url from the log $file:

>>>FILE base.txt text/plain
The request signature we calculated does not match the signature you provided. Check your key and signing method.

I’m using my own AWS credentials with my own buckets.

aws s3 cp base.txt s3://vcf-test/AAAatest2/

works just fine.

I wish it were written in the example, clearly, that for it to work one needs to set the right CORS permission for the target bucket.
Alas, or at least a clear link to