First request to a webhook

To create a webhook, it has to pass the following validation:

1. Receive a GET request onto the webhook URL and read a specific "challenge" field from the query.
2. Send this field back to where the request comes.

Can this be implemented in an Integromat app?

Yes, you can use the verification directive for this purpose.

The code would look something like this:

{
    "verification": {
        "condition": "{{if(query.challenge, true, false)}}",
        "respond": {
            "status": 200,
            "type": "json",
            "body": {
                "challenge": "{{query.challenge}}"
            }
        }
    },
    "response": {
        "output": "{{body}}"
    }
}

It's up to you to specify the verification condition so that the platform will be able to tell if the incoming message is a verification request or a regular webhook payload.