my_custom_message with username and password

How can i use {username} and {password} in this function?

//Modify the message sent to a user after being approved.
function my_custom_message( $message, $user ){
	$message = text text "\r\n";
	$message .= "{username}\r\n";
	$message .= "{password}\r\n";
        $message .= text text";
        return $message;
}

Wait your answer

Thanks a lot

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