Do you actually vote for a candidate or for a list?
It's a party list vote with an additional preference vote component.
It means that you are primarily voting for a party, but you can write in a candidate of your choice from that party list as well. If that candidate receives a high enough vote, that candidate then gets promoted up the list in each party.
It is only allowed to write in a candidate from the party you voted for.
If you vote for ÖVP and write in Lasar (FPÖ), your preference vote is invalid - because the party list vote trumps the preference vote.
If you just write in Lasar (FPÖ), but without marking any party with an [X], your preference vote will automatically count as a vote for the FPÖ.