When using the catalogue API, some indices are not public and queries restricted to authorised user. For authenticating and being authorised to query a restricted index, you must specify your username and password or API key.
If you are using curl, simply use the -u <username>:<password or API key> argument scheme.
curl -u mrossi:ABcdEF "https://catalog.terradue.com"
With opensearch-client tool, the username and password are passed this way
opensearch-client -a mrossi:ABcdEF "https://catalog.terradue.com" identifier