@joshua Thanks for answering
So this is not yet possible and all I can do is thumbs up the issue and wait.

So can I display the "My custom message", which was returned from the webhook, above the login form as the reason of the login fail?

Hi,
I have a webhook that could fail of several reasons. So I want to return an error message based on the reason and then display that message on the login page.
From my understanding to fail a webhook you have to return a non 2xx message. But when I return a 500 error code on the login page I receive this error message:
A validation error occurred during the login attempt. An event log was created for the administrator to review.
and when I look in the event log I see this

Failed to complete a login for Identity Provider [simple oidc].

The following errors occurred:

{
  "fieldErrors" : { },
  "generalErrors" : [ {
    "code" : "[WebhookTransactionException]",
    "message" : "My custom message"
  } ]
}

After I read some github issues and some forum posts here I am still not sure if this is possible.

Thanks,
Mihai