Webhook endpoint address within a plugin container

I had following issue: my motion detection device sends webhooks with http verb GET. Hassio only accepts webhooks with verb POST. Neither the device manufacturer nor Hassio will do anything to solve this issue.

I developed a plugin which listens on GET requests and will do the respective POST requests to the webhook endpoint api.

My issue is, what is the correct webhook endpoint address from within a plugin container? I tried different addresses and none of them worked. Sometimes I even get 401 although the webhook endpoint should be reachable without authorization.

What is the correct endpoint for webhook api within a container?

Kind regards