You can select a default contact for Jobs, Estimates, Purchases or Invoices for any company in Briefcase. To do so, Navigate to the company and under the people tab, select the icon to view the contact's profile. On the bottom left you’ll be able to see the default for the dropdown. This will allow you to select one of the following options:
Select the option relevant to the contact and then press done on the top right. Note that this change will only apply to any new jobs, estimates, purchases or Invoices and will not affect existing ones.