$shortcode = 'hi';

    function page_init()
    {
?>
        <form action='<?php echo admin_url( 'admin.php' ); ?>' method='post'>
            <input type='text' placeholder='Your shortcode(s)' name='shortcode'/>
            <input type="hidden" name="action" value="shortcode_plugin" />
            <input type='submit' value='Appliquer'>
        </form>
<?php

    }

    add_action( 'admin_action_shortcode_plugin', 'shortcode_plugin_admin_action' );
    function shortcode_plugin_admin_action($shortcode)
    {
        if(isset($_POST['shortcode']))
        {
            global $shortcode;
            $shortcode = $_POST['shortcode'];

            wp_redirect( $_SERVER['HTTP_REFERER'] );
            exit;
        }

    }

    add_action( 'admin_menu', 'shortcode_plugin_admin_menu' );
    function shortcode_plugin_admin_menu()
    {
        add_management_page( 'Shortcode plugin Test page', 'Shortcode plugin Test page', 'administrator', 'shortcode_plugin', 'shortcode_plugin_do_page' );
    }

    function instaAboveFooter()
    {
        global $shortcode;
        echo do_shortcode($shortcode);
    }
    add_action( 'get_footer', 'instaAboveFooter' );