Onyx works the same on cTrader as on MetaTrader: it reads your trades read-only, never trades or touches your money. The difference is that in cTrader the connector is a cBot (in MetaTrader it's an EA), but it does the same thing.
Steps
1In Onyx, on Connect account, pick the cTrader platform.
2Download the Onyx cBot (.algo file or the .cs source).
3In cTrader Desktop: paste the cBot into Automate, compile it and add it to your chart.
4Paste your Onyx API key into the cBot parameters and hit Play (with the global automation button on).
Confirmation
When the cBot reports, your account shows as connected and your stats appear in seconds.
Onyx Guardian and copy trading also exist for cTrader, as separate cBots, with the same settings as on MetaTrader.