Filters and $_POST (or $request post vars)

I want to automatically add variables to each incoming POST, so I thought filters would be helpful, however, I can't see a way to get to the POST vars in the filter class

PHP Code:

class Filters extends BaseConfig
{
 
 public $aliases = [
 'csrf'    => CSRF::class,
 'toolbar'  => DebugToolbar::class,
 'honeypot' => Honeypot::class,
 'postfilter' => PostFilter::class
 ];
 public $globals = [
 'before' => [
 // 'honeypot',
 // 'csrf',
 ],
 'after'  => [
 'toolbar',
 // 'honeypot',
 ],
 ];

 public $methods = [
     'post' => ['postfilter']
    ];

 public $filters = [];
} 


the PostFilters class:

PHP Code:

class PostFilter implements FilterInterface
{
    public function before(RequestInterface $request, $arguments = null)
    {
        $_POST['Maint_Userid'] = "plugh";
        $_POST['Create_Userid'] = "plugh";
        return $request;
    }

    public function after(RequestInterface $request, ResponseInterface $response, $arguments = null)
    {
        // Do something here
    }
} 


I've also tried:

PHP Code:

public function before(RequestInterface $request, $arguments = null)
    {
        $post_request = \Config\Services::request();
        $post_request->Maint_Userid = "plugh";
        $post_request->Create_Userid = "plugh";
        return $request;
    } 


PHP Code:

public function before(RequestInterface $request, $arguments = null)
    {
        $request['Maint_Userid'] = "plugh";
        $request['Create_Userid'] = "plugh";

        return $request;
    } 


(and a few other variations of the above)

It is adding the varaibles to the RequestInterface $request (from log_messages, I see this). Even the direct $_POST didn't add it to the post vars in the controller.
Where should I be doing this (or how do I make the filters see the $_POST vars and add to them)?
Thanks.