From your Integrations page, click on the Connect button under Chargebee, and on Continue from the page you were redirected to.
Pick your currency, and fill in your Namespace - you will find it in your Chargebee account URL, right before .chargebee.com
Here, it would be demo-account
Click on the generated link.
👉 You will always be able to access it later from the Configure Chargebee page of your Chargebee settings (under Api Keys).
Click on Add API Key...
...and create one with a Write, Full-access by selecting these options from the menus you will be taken through.
Give it a name of your design before saving it by clicking on Create Key.
And copy it from Chargebee…