form_submit_using_rest_api.php

add_action('fluentform/loaded', function ($app) {
    $app->router->post('/submit-form', function () use ($app) {
    try {
    $data = $app->request->get('data');

    $data['_wp_http_referer'] = isset($data['_wp_http_referer']) ? sanitize_url(urldecode($data['_wp_http_referer'])) : '';

    $app->request->merge(['data' => $data]);

    $formId = intval($app->request->get('form_id'));

    $response = (new FluentForm\App\Services\Form\SubmissionHandlerService())->handleSubmission($data, $formId);

    return $app->response->json($response);
    } catch (FluentForm\Framework\Validator\ValidationException $e) {
    return $app->response->json($e->errors(), $e->getCode());
    }
    });
});