The procedure is not entirely obvious (the whole "mail alias" infrastructure is a UX nightmare), but this should do the trick:
- Make sure you are inside TUNET. This can be achieved by one of these methods:
- Connect to TU Wien using VPN. There is some relevant info on the TU.it website.
- Go to TU Wien physically and log into Wi-Fi (eduroam, tunet or tunetguest). Please note that logging into eduroam at any other institution will not place you inside TUNET.
- Use one of the internet room PCs or public access kiosks strewn around TU. There should be a few in the library if you need one on a Sunday.
- Open the authentication portal at https://iu.zid.tuwien.ac.at/AuthServ.portal and log in.
- Click on "Mail Alias Studierende/Alumni" in the link list on the left. (I cannot link you directly to the page as the number in the URL is different for every person -- it's the "ZID OID", the object ID in TU.it's internal authentication database.)
- Type in the name you want to have in front of @student.tuwien.ac.at. It has to be either firstname.lastname (unless that's already taken by someone else) or firstname.someothertext.lastname. If I recall correctly, should you have multiple first or last names, the first first name and the last last name count.
- Press OK. Your alias will be set.
- Give the server about fifteen minutes to distribute this information to all the necessary places in the mail routing system.
Hope that helps.