https://stackoverflow.com/questions/33858927/how-to-obtain-the-chat-id-of-a-private-telegram-channel

Update

I found crazy solution :

1. login under your account at web version of Telegram : https://web.telegram.org
2. Find your channel. See to your url, it should be like https://web.telegram.org/#/im?p=c**1055587116**_11052224402541910257
3. Grab "1055587116" from it, and add "-100" as a prefix.

So... your channel id will be "-1001055587116". Magic happen :)