Where are registration and login?

Use login page to login the site,
https://staging.eggardon-colmers-view.org.uk/wp-login.php
You can add frontend login and register pages using third-party plugins

Thank you @chathuripxl

How would I change this page to have a link to the login page?

You can add a plugin or add code to redirect to login page if this is visited.

add_action( 'template_redirect', 'redirect_to_specific_page' );

function redirect_to_specific_page() {
    if ( is_page('add-edit-your-event') && ! is_user_logged_in() ) {
        wp_redirect( 'https://staging.eggardon-colmers-view.org.uk/wp-login.php', 301 ); 
        exit;
    }
}