I see. These are connection certificates. They establish a secure communication channel between two entities, basically a VPN - Virtual Private Network. Because it's private, it requires its own encryption/decryption keys which are provided by the certificate.
Unfortunately, AppSheet does not support this style of secured communications at this time.
However, I believe you could implement Google App Scripts to handle this. Your app calls a script, passing the necessary arguments and the script in turn builds the JSON payload and calls the API.
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