You can link an appointment to an opportunity within the
"Activities Tab".
Link an Appointment to an Opportunity
- Click on “Activities” to open your activity box.
- Click and select the particular appointment you want to link (it’s detail window will show-up at the bottom of the activities box).
- Within the Appointment Tools bar, click on the “Open” button.
- Click on Browse to search for entity to attach Appointment to.

- Search for the entity in the Look Up dialog and Select.

- Configure other settings for your appointment and click "Save".
Tip: You can link an appointment for a specific opportunity also within that opportunity itself. Open this opportunity (by double-clicking on it) and click the
"Activities". Once you create a new appointment for this opportunity, it will be automatically assigned to this it.
Since the version 11, it is also possible to link multiple Accounts/Contacts/Leads/Opportunities to the same activity.