API key is composed of GUID and Token separated by a colon:
To generate you GUID and Token, go in Clever Monitor application, in the menu select Administration/web services and click New. View detail of the new record to see GUID and Token.
Use the API key in the X-Api-Key HTTP header:
curl -H 'X-Api-Key: GUID:TOKEN' 'https://api.clevermonitor.com/v1.2'
It is necessary to send SNI (Server Name Indication) for successfull API authentication!