A supervisor can only be assigned to a professional after they have been confirmed for a job.
After you confirm the professional:
In the Web Portal: Click on the confirmed job in the Scheduled Jobs tab, then click on Assign Supervisor and choose the appropriate supervisor.
On the app: Tap on the confirmed job in the Scheduled Jobs tab, then tap the right arrow to view details about the candidate. Finally, click Assign Supervisor at the bottom of the screen to assign a new supervisor.