It's basically the same as GET and the others. The request object contains the url encoded form data or whatever was posted. The request object is the same as ZnRequest with a few additional methods. There is a generic at: method that can be used to access both path and queryOrFormParams.
Teapot on
GET: 'login' ->
'<form method="POST">
User name:<br>
<input type="text" name="user"><br>
Password:<br>
<input type="password" name="pwd"><br>
<input type="submit" value="Submit">
</form>';
POST: '/login'-> [ :req | 'Welcome ', (req at: #user) ];
start.