How to obtain the chat_id of a private Telegram channel

get_telegram_channel_id

You should convert it to public with some @channelName

Send message to this channel through Bot API

https://api.telegram.org/bot111:222/sendMessage?chat_id=@channelName&text=123

bot+[yoour bot id]

As response you will get info with chat_id of your channel.

{
  "ok" : true,
  "result" : {
    "chat" : {
      **"id" : -1001005582487,**
      "title" : "Test Private Channel",
      "type" : "channel"
    },
    "date" : 1448245538,
    "message_id" : 7,
    "text" : "123ds"
  }
}
Now you can convert Channel back to private (by deleting channel's link) and send message directly to this chat_id "-1001005582487"

https://api.telegram.org/bot111:222/sendMessage?chat_id=-1001005582487&text=123