We've noticed that you are using an unofficial Telegram app. You're totally free to choose the app you prefer, at your own risk. However, the instructions we give are based on the official app, which is free, shows no ads, and has the latest features first. The instructions for your particular app may differ, so if you can't find the right place to type in the code, you'll need to contact the app's creator for support.
Alternatively, you can switch to the official app. All your chats - apart from Secret Chats - will be safely transferred over, as they're stored in the Telegram Cloud. Get it from the Google Play Store:
https://telegram.org/dl/android