This is "Module 8 – Completing a Planner Task via the Web", which is part of a series on "How to Integrate Microsoft Project and Microsoft Planner".
53. Once a Planner task is complete, click on the task name, input the actual finish date as the Due date, then change the Progress value to Complete. Click on the x in the upper right to close
54. The task now appears within the Completed bucket
You may find this of value: