To add credit to a client’s wallet in WHMCS, follow these steps:

1. Log into the WHMCS Admin Area.
2. Go to Clients > View/Search Clients and select the desired client.
3. On the client’s profile page, click the Manage Credits tab.
4. Click on Add Credit.
5. Enter the amount of credit you want to add and an optional description.
6. Click Save Changes.

The credit will be added to the client’s account for future invoices or services.