This depends on where you use Duolingo. If you use the mobile app, it depends on the keyboards that you have installed on your device. Usually you can type most of them by long touching a letter. On my keyboard layout, however, the long o and the long y are missing.

If you use the web interface, there are small buttons for all those special letters directly beneath the text box where you enter your answer.

Duolingo will usually also accept a double vowel instead, if you can't type in those letters. In this example, "AAbra roovi azanti jorraaelza" should (!) be accepted, however, not all sentences correctly accept this at the moment.