To build your version of Telegram Desktop you're required to provide your own api_id and api_hash for the Telegram API access.
How to obtain your api_id and api_hash is described here: https://core.telegram.org/api/obtaining_api_id
If you're building the application not for deployment, but only for test purposes you can use TEST ONLY credentials, which are very limited by the Telegram API server:
api_id: 17349 api_hash: 344583e45741c457fe1862106095a5eb
Your users will start getting internal server errors on login if you deploy an app using those api_id and api_hash.