I have created Webhook and confirmed the ID by
https://trello.com/1/tokens/{token}/webhooks/
Board ID is identical with my board.
Callback URL is GAS (anyone, even anonymous) and my Web form can successfully post to the URL, then doPost is executed.
But, card modification or moving in Trello seems not to call the URL.
What kind of causes can you think of?
Or, Webhook is not available in free plan?