You just send a POST request to:
https://api.telegram.org/bot{token}/{method}
For example:
https://api.telegram.org/bot123456:ABC-DEF1234ghIkl-zyx57W2v1u123ew11/sendMessage
In the body of the request, you URL encode the parameters:
chat_id=12345&text=hello%20friend
For example, in Python using the requests module:
import requests
response = requests.post(
url='https://api.telegram.org/bot{0}/{1}'.format(token, method),
data={'chat_id': 12345, 'text': 'hello friend'}
).json()
When a user chats with your bot, you get a Message object that has a chat id (and a user id, which you can substitute for a chat id). There's no way to initiate a chat with a user unless you already know their user id, so you have to wait for a user to talk to you. You can simplify that by using deep linking and having the user click on a link that sends a pre-made message when they hit the Start button.
Edit: for those struggling to find chat_id, here's a way:
1.- Create a bot: on Telegram's search look for @BotFather. Click start, write /newbot, give it a name and a username. You should get a token to access the HTTP API. Save this token.
2.- Find your bot on Telegram with its username. Write something to it e.g. 'test'. This will come in handy later.
3.- Print chat_id. Before running this function, make sure that you have at least written one message to your bot on Telegram (step 2)
Javascript code:
var token = "123456:kioASDdjicOljd_ijsdf"; // Fill this in with your token
var telegramUrl = "https://api.telegram.org/bot" + token;
function getChat_id(){
var res = UrlFetchApp.fetch(telegramUrl+"/getUpdates").getContentText();
var res = JSON.parse(res);
Logger.log(res.result[0].message.chat.id.toString());
}
