Hi @AnnaJordan
If I am right, you need something like this:
You have to choose your plan, and create an item on a SharePoint list (I created one called Planner Tasks, which have Title and Due Date columns).
On the other hand, there is no update task planner trigger action, so you can't update the SharePoint list item when the planner task is updated, unless the task is completed or assigned to the user executing the flow. Currently, Planner trigger actions are the following:

Hope it helps!
Ferran