In Part 2, there are some options presented on how to generate the encoded string for authentication. If you are looking to validate the authentication using cURL on the command line, you will need an external script or utility to manually generate the encoded string, and then copy the final result into your cURL command.
I do have the public certificate (.pem) and encoded strings provided. That's why I thought having the cURL on the command line with those info would be able to work. Not really sure what went wrong there.
If you have the 3 required strings (CA cert text, encodedData, and encodedSignedData), this should work using cURL. The certificate goes in the header ("-H") and the other two strings are going in the request body ("-d") in JSON format (enclosed in curly brackets).
Note that sometimes, the CA cert text will have "begin" and "end" tags in the file, but you'll need to strip these out.