Although Hitler won the referendum (90% for, 10% against), the result was a fraud; and he had effectively already assumed these powers on the death of Hindenburg, some two weeks earlier.
From Wikipedia:-
Voters were asked the question:
The office of the President of the Reich is unified with the office of the Chancellor. Consequently all former powers of the President of the Reich are transmitted to the Führer and Chancellor of the Reich Adolf Hitler. He himself nominates his substitute.
Do you, German man and German woman, approve of this regulation provided by this Law?
The government used widespread intimidation and electoral fraud to secure a large "yes" vote. This included stationing storm troopers at polling stations and forcing clubs and societies to march to polling stations escorted by Nazi storm troopers and then to vote in public.
In some places, polling booths were removed, or banners reading "only traitors enter here" hung over the entrances to discourage secret voting. In addition, many ballot papers were premarked with "yes" votes, spoiled ballot papers were frequently counted as having been "yes" votes and many "no" votes were recorded to have been in favour of the referendum question.
The extent of the fraud meant that in some areas, the number of votes recorded to have been cast was greater than the number of people able to vote.