override-default-admin-message.php

/**
 * Modify the contents of the message sent to the admin when a user signs up.
 * 
 * @param $message The default message.
 * @return string the updated message.
 */
function my_custom_message( $message ) {
	$new_message = 'USERNAME (USEREMAIL) has requested a username at SITENAME' . "\n\n";
	$new_message .= "SITEURL\n\n";
	$new_message .= 'To approve or deny this user access to SITENAME go to' . "\n\n";
	$new_message .= "ADMINURL\n\n";
    
    return new_message;
}
 
// modify the message sent to the admin
add_filter( 'new_user_approve_request_approval_message_default', 'my_custom_message' );