Calendly and Cal.com
Send your own booking page, and bookings made there come back into Elsin as interviews.
Each member connects their own account. A booking is matched to the invite link, or otherwise to the candidate's email, and creates, moves or cancels the Elsin interview.
Calendly
Paste a Calendly personal access token. Elsin subscribes to new and cancelled bookings itself. Calendly only sends bookings back on its Standard plan or above; on its free plan the link still works but bookings do not return.
Sign in with Calendly is not set up, so use the token.
Cal.com
Paste an API key, for Cal.com's cloud or your own server's API v2 address. Elsin tries to create the webhook. If it cannot, open Settings, Developer, Webhooks in Cal.com and add one with Elsin's address and secret for booking created, rescheduled and cancelled.
Elsin's own booking links need neither tool: see scheduling.