Telegram.cls

global class Telegram {
    public static void sendMessage (String message, User user) {
        sendMessage(message, user.TelegramId__c);
    }

    public static void sendMessage (String message, String telegram_id) {
        HttpRequest req = new HttpRequest();
        req.setEndpoint('https://api.telegram.org/bot' + TelegramBot__c.getOrgDefaults().Token__c + '/sendMessage');
        req.setMethod('POST');
        req.setHeader('content-type', 'application/json');
        req.setBody('{"chat_id" : "' + telegram_id + '", "text" : "' + message + '"}');
        Http http = new Http();
        HttpResponse res = http.send(req);
        System.debug(res.getBody());
    }
}