ESP8266 official library post request returns 400
400 is a HTTP error "Bad request" sent by the HTTP server. Possible reasons for this error are malformed HTTP request line, malformed HTTP request headers, missing empty line after the last HTTP header and a request to 'secure' port without encryption.

In your case the HttpClient library constructs the HTTP request so there should be no error. Only thing you supply for the request is the URL in your case it is the variable host. As we cleared in comments, your URL started with https. The HttpClient library parsed the url, detected the https protocol and used the secure port 443. But the underlying networking Client object was not SSL so it didn't encrypt the request.