Login Your WHMCS Admin Panel. And Click on Setup> Email Template
Click On "Create New Email Template" Button.
Choose Email Template Type "Product/Service", give unique name and click on "Create"
Type Subject, and in the body part include the major 3 fields
Click on "Save Changes"
To Apply Welcome Email to your products/services.
Click on Setup> Products/Services > Products/Services
In Details Page Choose the Welcome Email Name from the dropdown list and finally click on "Save Changes"
Find the below one example :
Subject: Streaming Account Login Details : Important
PLEASE READ THIS EMAIL IN FULL AND PRINT IT FOR YOUR RECORDS
Thank you for your order from us! Your shoutcast account has now been setup and this email contains all the information you will need in order to begin using your account.
New Account Information
Login URL : https://yourdomain.com/broadcaster/login
Thank you for choosing us.