How can I create a follow up activity on a contact and link it to a prospect/customer?

 Open your contacts. Select the contact and click on the ‘Follow up’ button.

Select the ‘Layout’ button and ‘Other fields’.

 Select ‘Contacts’ and unfold this selection. Scroll down and select the 'Customer' or 'Prospect' fields required. Click on the large black arrow to the right of the field list and click on ‘Finish’.