We post directly to the google spreadsheet from a custom form and do not use google forms; the code snippet below is in python and posts to the google sheet which is then captured via google sheet's doPost(e) function:

SiteFormToGSheet.pyc

form_fields = { "fname": firstName, "lname": lastName, "title": title, ... rest of form fields to go to spreadsheet}
url = "https://script.google.com/macros/s/[your spreadsheet key]/exec"
form_data = urllib.urlencode(form_fields, doseq=True)
result = urlfetch.fetch(url=url, payload=form_data, follow_redirects=True,method=urlfetch.POST,
                headers={'Content-Type': 'application/x-www-form-urlencoded'})