Go to the Marketing app, and select the Engagement à Journey section.
Considering Our scenario is to send WhatsApp Message whenever the user registers for an event from Event Portal.
Click on New Journey from the Ribbon button.
Now select Name and Trigger-based.
Here select the custom trigger created earlier i.e, WhatsApp integrate.
Set the timezone, Start, and end Date in the right side pane.
Click the + symbol to add an action and then. select Add a custom trigger.
Select a custom trigger and Map attributes for Body and PhoneNumber.
Click Map Attribute. It prompts popup as the Set trigger field; you choose the attribute or value as per the requirement.