Add additional data to request object after initialisation - php

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#email.com");
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));