Using subdomain as trello webhook endpoint

I am running a server where I have Node-Red installed and I want to set up a webhook, so I can receive data when there is an update on a trello board.

My problem is that I can set one up with the callbackURL “http://public-ip:port”, but not “http://public-ip:port/webhook”.

With “/webhook” or any other subdomain it gives me this: VALIDATOR_URL_RETURNED_ERROR

Something to note, is that both URLs work when I use “curl -X POST” command from the CMD on windows.