Grafana Alerting does not offer SignalK integration for notifications, and it looks like SignalK has the special format of the payload in the notifications API. Therefore, the webhook integration will not work out of the box. Also, it does not support sending authentication requests. Instead, it expects either basic authentication or a token that would work in PUT request.
So, the only recommendation I can give you is to run a webhook service on your side (for example an Azure Function, or AWS Lambda) that would accept requests in Grafana webhook format, convert it to SignalK format, submit a request to SignalK.