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…

…into Upflow.