You are missing that is a Authorization Header - Scheme and not header key, so you are sending header:
Authorization: x-api-key <API-KEY>
but you need header:
x-api-key: <API-KEY>
Webhook doesn’t support custom headers.
I would recommend to create custom authorizer (Lambda) in your API GW, where you will be checking basic auth, because basic auth is supported by Grafana webhook.