Do I need to have a valid return ticket when entering Austria on visa type D? [closed]

I am applying for a visa type D for Austria at the Austrian embassy - this will allow me to stay up to 180 days in Austria. My plan is to go to Austria and marry my fiancée there and then request residency rights in Austria and live with my fiancée.

My visa will be sponsored by my fiancée, who has a job, an apartment and can support me.

Can I simply buy a one way ticket to Austria from my non Schengen country or do I need a valid return trip when embarking my flight to Austria?

For the visa purposes I am using a "dummy" flight reservation, as advised by the embassy itself, so in case I am not granted the visa I do not lose the money from a real flight booking.

With dummy flight reservation I mean I have a reservation for a round trip flight that is refundable and/or I have not yet paid for but it is locked/reserved in my name