The first thing you need to do is login to your WHM and generate an API token.
Search for "api" to quickly locate the API tokens link
Generate a new token
Give your new token a name, scroll down and save
Save your API token somewhere in your computer. You won't be able to retrieve it from WHM later.
In Hosting Billing, click on Servers, Add Server, and fill in the details with your server details as shown below: