A common use-case for sending an SMS template to a lead with an action would be to follow up with a lead that you were not able to connect with over the phone.
After you have created the necessary SMS template, the following instructions will help you configure the action-based automation:
Navigate to More > Settings > Sales Workflow.
Click the Edit button (the pencil icon) for the workflow containing the status that your action is available for.
Scroll down to find the correct status, then click the Settings button (gear icon) for the action that you want to configure to send an SMS template.
In the Operations settings, enable the Send SMS Template operation and select the SMS template that should be sent to the lead when the action is used.
Click the green Update Action button to save your configuration