For this, have to follow some steps.
First, log in to your WHMCS Admin Panel. Then, go to Setup> Email Template.
Click On the "Create New Email Template" Button.
Then choose Email Template Type as "Product/Service", give a unique name, and click on "Create".
Then, type Subject, and in the body part including the major 3 fields
And click on the "Save Changes" button to save.
How To Apply Welcome Email to your products/services?
First, click on Setup> Products/Services > Products/Services
In Details Page choose the Welcome Email Name from the dropdown list and click on the "Save Changes" tab.