I'm currently triggering my Jenkins builds through a GitHub webhook. How would I parse the JSON payload? If I try to parameterize my build and use the $payload variable, the GitHub webhook fails with the following error:

```
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"/>
<title>Error 400 This page expects a form submission</title>
</head>
<body><h2>HTTP ERROR 400</h2>
<p>Problem accessing /job/Jumph-CycleTest/build. Reason:
<pre>    This page expects a form submission</pre></p><hr /><i><small>Powered by Jetty://</small></i><br/>                                                
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</body>
</html>

```

How can I get my GitHub webhook to work with a parameterized Jenkins build, and how could I then parse the webhook payload to use certain lines, such as the username of the committer, as conditionals in the build?