You can manually start a work order task in the Rental Mobile Operations - Field Service app.
Note:
If you are working offline, the status change is not synchronized with the work order line status in D365 F&SCM until you are back online.
Steps
On the Booking page, tap the status drop-down list.
Tap 'In Progress'.
Tap the floating action button > Save.
Notes
Usually, to start a work order task, you use Start Travel or Start Work.