Android 6 allows third party developers to connect to the fingerprint sensor via API. Telegram supports unlocking the passcode with the fingerprint. You have to set up a fingerprint in the Android settings.
Samsung Galaxy S6 and Samsung S6 Edge
- Launch the Settings app and tap on “Lock screen and security” under the Personal tab.
- Tap on Fingerprints, located in the middle of the screen just under the Security heading.
- First-time users will immediately be prompted to scan a fingerprint.
Those who already have at least one fingerprint registered will need to tap on “Add fingerprint” and follow the prompts.
CNET has an excellent how-to with screenshots for the S6 here.