How to get a custom page through plugin?

You can use wp_insert_post for creating the page.

<?php
    $content="<Sign up form content>";
    wp_insert_post(array(
        'post_title' => 'My page',
        'post_content' => $content,
        'post_type' => 'page', //VERY IMPORTANT
        'post_status' => 'publish/draft'
    ));

Check it.