Source

File: wp-includes/general-template.php

function wp_registration_url() {
	/**
	 * Filters the user registration URL.
	 *
	 * @since 3.6.0
	 *
	 * @param string $register The user registration URL.
	 */
	return apply_filters( 'register_url', site_url( 'wp-login.php?action=register', 'login' ) );
}