One calendar ID per line. Each must be public and each must be allowed by the same API key.
The calendar must be public: Google Calendar → Settings for that calendar → Access permissions → Make available to public. A private calendar returns zero events even with a valid key.
If Google says the referer is blocked, add this site to the key’s website restrictions in Google Cloud → Credentials.
Sync (JSONBin)
Enter the master key, then Create bin on one device. Copy the bin ID to your other device and enter both there.