"Nya" in this case is the definite form.
Adjectives that directly modify definite nouns like this ("Det [adj] [noun]et"/"Den [adj] [noun]en") always take the definite form.
In nearly all cases, the definite form of an adjective is exactly the same as the plural form.
One of the notable exceptions is "Liten", which becomes "Lilla" in the definite form instead of "Små" like the plural form.
I answered, "Have you the new password?" I agree that not many people would say it that way, but there's nothing wrong with it.