"Please follow the rules of our site, click accept button to agree and login in:"

                                   [Accept Button]

mydata.redirect_form:
       path: '/user/redirect'
       defaults:
         _form: 'DrupalmydataFormRedirectForm'
         _title: 'Redirect User'
       requirements:
         _permission: 'access content'

use SymfonyComponentHttpFoundationRedirectResponse;
use DrupalCoreUrl;

function mydata_preprocess_page(&$var){
    if(empty($_SESSION['terms_accepted']) || $_SESSION['terms_accepted'] != 'ok'){
        if (Drupal::routeMatch()->getRouteName() == "user.login"){
            $path = Url::fromRoute('mydata.redirect_form', [], ['absolute' => TRUE])->toString();
            $response = new RedirectResponse($path);
            return $response->send();

        }
    }
}

namespace DrupalmydataForm;

use DrupalCoreFormFormBase;
use DrupalCoreFormFormStateInterface;
use DrupalCoreUrl;
use SymfonyComponentHttpFoundationRedirectResponse;

class RedirectForm extends FormBase {

    public function getFormId() {
        return 'redirect_form';
    }

    public function buildForm(array $form, FormStateInterface $form_state) {
        if(!empty(@$_SESSION['terms_accepted']) || @$_SESSION['terms_accepted'] == 'ok'){
            $path = Url::fromRoute('user.login', [], ['absolute' => TRUE])->toString();
            $response = new RedirectResponse($path);
            return $response->send();
        }

        $form['test'] = [
            '#type' => 'item',
            '#markup' => t('Please Follow the rule of our site,click accept button to agree and login-in: '),
        ];

        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Accept',
        ];

        return $form;

      }

    public function submitForm(array &$form, FormStateInterface $form_state) {
        $_SESSION['terms_accepted'] = 'ok';
        if($_SESSION['terms_accepted']){
            $form_state->setRedirect('user.login');
        }
    }
}