Additionally.
Each Webhook’s topic you can test if you can subscribe to the topic by making a fetch call to the topic as each topic has a corresponding helix end point.
So if you can fetch https://api.twitch.tv/helix/subscriptions/events?broadcaster_id=123456789&first=1 then you can create a webhook subscription for it. (easy way to test your token is correct without making a subscription and waiting for data to occur or a denied response)