A template message (also known as a Highly Structured Message or HSM) is a special message type that must be used for any business-initiated or re-engagement conversations via the WhatsApp channel.
In order to initiate a conversation as a business with an end-user for the first time or after a period of 24 hours since the last interaction, you have to make use of a pre-approved template. WhatsApp only permits freeform text when you are responding within 24 hours of a customer reaching out to you. Templates can only be sent to users who have opted in and given your app permission to send them messages.
Please read more regarding the Opt-In rules in the WhatsApp Business Policy.
You can easily submit HSMs that are specific to your business from within the Clickatell user portal. Your templates are first reviewed by Clickatell, and then by WhatsApp. Once approved, you’ll be able to specify the template ID and the relevant parameters in your API call when sending messages or transactional notifications to your customers.
For more on template messages and how to create them in your portal, see here.