Outlook has a useful feature called Contact Linking. It enables you to link a contact with an appointment, task, other contacts, or email. Prior to Outlook 2007, the forms had a field with a Contacts button. In Outlook 2007 and Outlook 2010 you needed to go into Contact Options and enable a setting to see the Contact Linking field. In Outlook 2013 and 2016, this option is gone.

You can still link Contacts in File > Properties of an open item but that is an extra step.

Fortunately, there is a way to enable the Contact Linking field: edit the registry.

Outlook 2016 user key
HKEY_CURRENT_USER\​Software\​Microsoft\​office\​16.0\​outlook\​preferences
DWORD: showcontactfieldobsolete
Value = 1
Outlook 2013 user key
HKEY_CURRENT_USER\​Software\​Microsoft\​office\​15.0\​outlook\​preferences
DWORD: showcontactfieldobsolete
Value = 1
Outlook 2016 - Group Policy
HKEY_CURRENT_USER\​Software\​Policies\​Microsoft\​office\​16.0\​outlook\​preferences
DWORD: showcontactfieldobsolete
Value = 1
Outlook 2013 - Group Policy
HKEY_CURRENT_USER\​Software\​Policies\​Microsoft\​office\​15.0\​outlook\​preferences
DWORD: showcontactfieldobsolete
Value = 1