Are you running this from the browser or desktop application?
If you run it from the desktop application, it should remove CORS as a potential issue.
For troubleshooting purposes, if you open the request in Postman and look at the right hand side of the GUI, you should have code snippet options.
Try running the request using CURL.
If it fails there too, then the issue isn’t related to Postman.
Can you clarify the bit about the supplier website. Are you saying they have a website service where you enter your credentials and it will test the API connectivity and that is also failing?
I can only re-iterate that “Service Temporarily Unavailable” errors are usually a supplier issue.
It means that you are hitting the server, but the service is not responding correctly. (Otherwise I would be advising to check the local firewall settings).The supplier should be able to see server side logs when this happens which will give them information on why its failing.
If you were sending the wrong details, you would normally get an 4xx authentication error instead.