Not sure if this is to do with the number of headers being provided or something else but when using the below, it returns a 200 ok response as expected:

However when adding even 1 more header, it returns response: BadRequest. Http request failed: the content was not a valid JSON.

I've tried the same request in Postman with the same headers and it appears to work fine so I assume it's something to do with the custom connector - the definition of this action in the custom connector seems fine and wouldn't have thought there would be an issue there as surely if there was a syntax error or something then it would allow me to perform the action at all.
Any ideas?