Step::1 Login to your WHMCS.
Step::2 Hover over the Setup section and go to products/services option and click on Products/Services.
Step::3 Now in the next page click on the Edit icon of the product.
Step::4 Then jump to the Free domain tab and fill out the details.
Step::5 Click on Save Changes.
That's It !Check out the video tutorial to get in detailed procedure of the above mentioned steps. You may read the blog as well.