You must be a verified user to make API requests. You can authorize against the API using the API token found in your Partner portal.
Once you have your API key you will need to supply it in every request as a query string parameter.
Example:
curl -v -H "Content-type: application/json" -H "Accept: application/json" -X GET https://platform.sectigo.com/users/48767?api_key=012c513e6d309c158a19bcbc86bb7a88