yii2-app-basic/tests/_pages/LoginPage.php

<?php

namespace tests\_pages;

use yii\codeception\BasePage;

class LoginPage extends BasePage
{
	public $route = 'site/login';

	/**
	 * @param string $username
	 * @param string $password
	 */
	public function login($username, $password)
	{
		$this->guy->fillField('input[name="LoginForm[username]"]', $username);
		$this->guy->fillField('input[name="LoginForm[password]"]', $password);
		$this->guy->click('#login-form button[type=submit]');
	}
}