You can do this by making use of the get Updates method
Entering the below code into any web browser immediately after adding the bot to the group. You will be able to locate the ID from the resultant JSON response.
<https://api.telegram.org/bot123456:ABC-DEF1234ghIkl-zyx57W2v1u123ew11/getUpdates>
Note that the group ID will always be a negative value.
20% project Telegram group details