To assign a default employer contact, go to Employers and choose from one of the employers in the list.
This takes you to the employer-specific dashboard. From here click the pencil icon to edit the employer:
Click on the Contact in the edit screen:
Then type the name of the contact. If they're in the list already, click on their name. If not, type their full name and click the orange +.
Next enter their email
Finally click Invite. On the following screen, there'll be an option to automatically reassign all apprentices from that employer to this new contact. Make sure it's checked before you hit Submit. The supervisor will be invited to WorkHands and any Bulk Entry will automatically assign this supervisor from now on.