How do I set the Post field after manipulation
If you're just looking to update a custom field on the user, something like this should do it:

craft()-&gt;on('users.onBeforeSaveUser', function(Event $event) {

    if(craft()-&gt;request-&gt;isSiteRequest())
    {
        $companyId = craft()-&gt;request-&gt;getPost('email');
        $event-&gt;params['user']-&gt;getContent()-&gt;companyId = $companyId;
    }

});