On Windows each crash should produce a minidump file in the tdumps folder near the telegram.exe file. This files contain useful info for the app developer. But before sending such files, contact Telegram Support chat and tell them reproduction steps. Maybe the issue is already known.
Detailed crash reports are available in the public alpha version. In case you run into a crash, collect the crash ID and send it to Telegram Support chat with the exact steps.
On macOS / OS X you get a crash report right after it happened and you need to save it to some text file and send as an attachment to Telegram's support chat volunteers.