Consent Forms must first be uploaded to Patients > Consent Forms before they can be connected to specific appointments. Related: Consent Forms: Uploading and Setting Up New Forms
1. Click on the Appointment on the calendar.
2. Click in to the "Consent Forms" box, where you can select which Consent Forms would be available to the patient checking in for this appointment.
To avoid manually attaching Consent Forms to each individual appointment, consider attaching them to an Appointment Profile: How do I add consent forms to an appointment profile?