Add additional data to request object after initialisation

Question

The below public function returns oauth token against user name and password. However, I have a requirement where, the username has to queried first from email id. In the first part of the function, I need to somehow add the username to the request object. The request is created using laminas from what I can understand.

Full code from which function is taken is here.

  /**
   * Processes POST requests to /oauth/token.
   */
  public function token(ServerRequestInterface $request) {

    ////////////////
    //////////////// 
    // ADD LOGIC TO GET EMAIL FROM REQUEST & GET USERNAME
    // ADD USERNAME TO $request
    ////////////////
    ////////////////
    
    //Extract the grant type from the request body.
    $body = $request->getParsedBody();
    $grant_type_id = !empty($body['grant_type']) ? $body['grant_type'] : 'implicit';
    $client_drupal_entity = NULL;
    if (!empty($body['client_id'])) {
      $consumer_storage = $this->entityTypeManager()->getStorage('consumer');
      $client_drupal_entities = $consumer_storage
        ->loadByProperties([
          'uuid' => $body['client_id'],
        ]);
      if (empty($client_drupal_entities)) {
        return OAuthServerException::invalidClient($request)
          ->generateHttpResponse(new Response());
      }
      $client_drupal_entity = reset($client_drupal_entities);
    }
    // Get the auth server object from that uses the League library.
    try {
      // Respond to the incoming request and fill in the response.
      $auth_server = $this->grantManager->getAuthorizationServer($grant_type_id, $client_drupal_entity);
      $response = $this->handleToken($request, $auth_server);
    }
    catch (OAuthServerException $exception) {
      watchdog_exception('simple_oauth', $exception);
      $response = $exception->generateHttpResponse(new Response());
    }
    return $response;
  }

The request is send as form data: See example js code below: (username is accepted, email param is added to demonstrate whats needed)

var formdata = new FormData();
formdata.append("grant_type", "password");
formdata.append("client_id", "828472a8-xxxx-xxxx-xxx-ab041d3b313a");
formdata.append("client_secret", "secret-xxx-xxx-xxx");
//formdata.append("username", "username");
formdata.append("email", "[email protected]");
formdata.append("password", "password");

var requestOptions = {
  method: 'POST',
  body: formdata,
  redirect: 'follow'
};

fetch("{{base_url}}oauth/token", requestOptions)
  .then(response => response.text())
  .then(result => console.log(result))
  .catch(error => console.log('error', error));