set-wp-registration-link-for-login-form-nav.php

<?php
/**
 * Set the registration link to the WordPress registration page on the PMPro login form.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function wp_registration_login_form_nav( $links, $allowed_html ) {
	// Is anyone can register checked (Settings > General)
	if ( ! get_option( 'users_can_register' ) ) {
		return $links;
	}

	// Set registration link to the WordPress registration page.
	$links['register'] = sprintf( '<a href="%s">%s</a>', esc_url( wp_registration_url() ), esc_html__( 'Register', 'paid-memberships-pro' ) );

	return $links;
}
add_filter( 'pmpro_login_forms_handler_nav', 'wp_registration_login_form_nav', 10, 2 );