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.
HKEY_CURRENT_USER\Software\Microsoft\office\16.0\outlook\preferences DWORD: showcontactfieldobsolete Value = 1
HKEY_CURRENT_USER\Software\Microsoft\office\15.0\outlook\preferences DWORD: showcontactfieldobsolete Value = 1
HKEY_CURRENT_USER\Software\Policies\Microsoft\office\16.0\outlook\preferences DWORD: showcontactfieldobsolete Value = 1
HKEY_CURRENT_USER\Software\Policies\Microsoft\office\15.0\outlook\preferences DWORD: showcontactfieldobsolete Value = 1