WHMpress works by caching your prices, domains lists and lots more into your WordPress installation, so that it does not have to fetch it from WHMCS every time.
During the setup, WHMpress syncs with WHMCS database. In this syncing process, WHMpress fetches necessary data from WHMCS database, so you need to provide its relevant details.
Although some expert users may choose to create an additonal WHMCS database user for this purpose, but you can also use existing database information which WHMCS is itself using. The best place to find this information is configuration.php file located in your WHMCS folder. Following are the necessary details you require from this file to sync WHMCS to your WordPress site.