My guess is you need to set the custom headers. BE has a function for that. I haven't looked at the Google Calendar API, but anything else I've done anything with require me to set the custom headers first.
Your URL is invalid, try removing "httppost://"
And, your usage seems not in API doc. Where did you get it?
I found "quickadd" as
to me, it looks the second https: could be the problem. My web service is not secure, so I don't know how it might work for you. Here's what my variable looks like (used in Insert from URL).
"&countWide=" & $$puzzleWidth &
"&countHigh=" & $$puzzleHeight &
"&path=" & $sourceImagePath
(I'm not sure if the method variable is required.)