How to change the registration URL

Jannah theme uses the standard WordPress wp_registration_url() function which provides the register_url filter to allow plugins to change the link, check this link for more example

If you want to change the default link, Add this code to your child theme’s  functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s functions.php file, as this will be wiped entirely when you update the theme.

add_filter( 'register_url', 'change_my_register_url' );
function change_my_register_url( $url ) {
    if( is_admin() ) {
        return $url;
    return "/custom-register-url/";