Show contact (link) in Outlook calendar entry, task, ...

Outlook has a nice feature called "Contact Linking" that allows you to link a contact with any appointment, task, other contacts, or even email. Adding this link is done with a button on the bottom of the appointment details.

In the new versions of Outlook (2013, 2016) this button is hidden by default, to reactivate it you have to change/add an entry in the registry, which are described here:

Outlook 2016:
HKEY_CURRENT_USER\​Software\​Microsoft\​office\​16.0\​outlook\​preferences

DWORD: showcontactfieldobsolete

Value = 1

Outlook 2013:
HKEY_CURRENT_USER\​Software\​Microsoft\​office\​15.0\​outlook\​preferences
DWORD: showcontactfieldobsolete
Value = 1

Find more details on this eg. on this page:
https://www.slipstick.com/outlook/contacts/show-contact-linking-fields/