To configure the CashU module in WHMCS, begin by activating it in Setup > Payments > Payment Gateways
You will then be asked to enter your Merchant ID and the Encryption Keyword.
You can find the Encryption Keyword inside your CashU account by going to Merchant Services > Service Setup.
This is where you also need to set the return url, which must be set to:
The Notification URL and all other fields should be left blank.