A trigger HAS to be the first element of your workflow.
What is the Contact List Trigger?
The Contact List Trigger adds contacts to your workflow. It is the starting point of your workflow.
How to use it?
Place the List block in your workflow
Select the contact list to start your workflow with
Select the type of inclusion:
Current contacts: all past contacts meeting the criteria of your list will enter the workflow at the time of publication;
Next contacts: only future contacts meeting the criteria of your list will enter the workflow (after publication);
Both: all contacts, current and future, will enter the workflow
4. Finally, when the next action is created, do not forget to link the List to this Action.