For creating tasks in Planner Premium, you can use the Add a new row action, from Dataverse connector, and reference the msdyn_projecttasks table:
This is a premium connector, so you will need a PA Premium license to use it.
In you question, you ask about 'which trigger to use': for creating a new task, you don't use any triggers, just the action mentioned above. You may use a trigger if you want to monitor the Project Tasks table to check if a new task was created/modified/deleted - in this case, you will use the When a row is added, modified or deleted and reference the same table.
Let me know if it works for you or if you need any additional help!
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