Endpoint not found error when trying to register a Target

I have been trying to register a target for a webhook but keep getting this error, Endpoint not found.

I am working through Postman and adding my PAT and BaseUrl.

Replacing https://example.com on the body page with an endpoint at webhook.site to see if it was something to do with my Logic Apps endpoint but I get the same error.

I tried with and without the body headers as well.

I hope you guys can provide some insight!