I did it manually. POST subscription gives me:
{"data":[{"id":"XXXX", "status":"webhook_callback_verification_pending", "type":"channel.follow", "version":"1", "condition":{"broadcaster_user_id":"116528496"}, "created_at":"2021-03-25T13:35:03.265284455Z", "transport":{"method":"webhook", "callback":"https://script.google.com/macros/s/XXXX/exec/webhooks/callback"}, "cost":1}], "limit":10000, "total":4, "max_total_cost":10000, "total_cost":1}
And got POST from Twitch:
"{
""subscription"": {
""id"": ""87b328e7-bbaf-4168-92af-ab9663f3928c"",
""status"": ""webhook_callback_verification_pending"",
""type"": ""channel.follow"",
""version"": ""1"",
""condition"": {
""broadcaster_user_id"": ""116528496""
},
""transport"": {
""method"": ""webhook"",
""callback"": ""https://script.google.com/macros/s/XXXX/exec/webhooks/callback""
},
""created_at"": ""2021-03-25T13:35:03.265284455Z"",
""cost"": 1
},
""challenge"": ""XXXXXXX""
}"
Then I POST to get all subscriptions and get:
{"id":"87b328e7-bbaf-4168-92af-ab9663f3928c", "status":"webhook_callback_verification_failed", "type":"channel.follow", "version":"1", "condition":{"broadcaster_user_id":"116528496"}, "created_at":"2021-03-25T13:35:03.265284455Z", "transport":{"method":"webhook", "callback":"https://script.google.com/macros/s/XXXX/exec/webhooks/callback"}, "cost":0}], "limit":10000, "max_total_cost":10000, "total_cost":0, "pagination":{}}
So there is already webhook_callback_verification_failed.