You can set a preferred contact in two ways.
Start by going to your Address Book, and clicking a subcontractor’s company from your list.
In the slideout that appears from the right, look at the Contacts table at the bottom. Then you have two options:
Option 1: Add a new preferred contact to a company
Click the ’+ Add Contact’ button.
Fill in the contact details, and tick Set this person as a preferred contact before you click [Add Contact] at the bottom.
Option 2: Set (or unset) an existing person as a preferred contact
Still in the subcontractor's company slideout, click the three dots menu icon for a person in the Contacts table.
Choose Set preferred contact from the menu that appears.
If you change your mind later, this menu option becomes Unset preferred contact.