How to autofill webform from GET vars?
Webform has GET support built-in, see: Setting Default Values from the URL.

For POST you have access to %post[key] or you can retrieve $_POST and set your values using hook_form_alter().


  Webform allows several special values to be used as defaults when
  configuring a component. The special placeholders are commonly called
  tokens. The tokens Webform supports include:
  %username
  %useremail
  %site
  %date
  %server[key]
  %session[key]
  %get[key]
  %post[key]
  %request[key]
  %profile[key]


Working with Default Values