This is only available on the new candidate profile. If your account is still on the old profile layout, you won't see the Custom Activity option when scheduling, this isn't a bug. See What's Changing: The New Candidate Profile for how to switch over.
Scheduling a meeting from a candidate profile used to mean two separate steps: create the event, then go back and log an activity for it. Now you can attach a custom activity right when you schedule, and it logs itself automatically, no second trip required.
Attach a custom activity while scheduling
Open a candidate's profile (new profile layout) and click the Schedule icon from quick actions.
Pick the date, time, and other event details as usual.
In the Custom Activity field, choose the activity type you want logged alongside this event, for example Reference Check or Client Debrief.
Click Save event. The invite goes out, and the custom activity is logged automatically on the candidate's timeline at the same time.
Which activities show up in the dropdown
The Custom Activity dropdown shows every custom activity type set up for candidates on your account, not just the ones currently enabled in your personal Activity Customisation view. See How to Create Custom Activities if you don't see the type you're looking for.
Practical example
A recruiter schedules a 30-minute reference check call from a candidate's profile. While filling in the event details, they select Reference Check from the Custom Activity field and save. The invite goes out, and the moment the event is created, a Reference Check activity appears on the candidate's timeline automatically, dated to the event. Nothing else to log afterward.
Things to know
Only available on the new candidate profile. On the old profile layout, this option doesn't appear.
Attaching a custom activity is optional. You can still schedule an event without one, the same way you always could.
This is currently only available when scheduling a new event from a candidate profile's Schedule tab, not when editing an existing event from the Calendar Widget. See Troubleshooting: Custom Activity Not Attached or Not Showing on a Calendar Event.
For what happens to the activity after the event is edited or deleted, see How Calendar Activity Mapping Works.