Yes this is a POST URL endpoint... I **bleep** around with it a bit last night and it seems that the problem is the curly brackets { } , because I tried removing those to get to the base post endpoint URL and it seemed to work in terms of the "trigger" accepting it, though I didnt actaulyl try sending any webhooks because that wouldnt work in my back end...
The curly brackets are baked into the POST endpoint URL, so I am not sure why it wouldn't work in HUbspot, but works in other applications.... Any thoughts there?