You don't need Roman numerals to write the name of this ionic compound.
As far as naming ionic compounds goes, Roman numerals are used to describe the oxidation state of the cation.
This is needed for elements that can form multiple cations, such as transition metals.
In your case, your cation is actually a polyatomic ion formed when an ammonia molecule forms a dative bond with a hydrogen ion.
The extra proton gives the ammonium ion its
What this ultimately means is that the ammonium ion can only have a
As a conclusion, any salt that contains the ammonium cation will not be written using Roman numerals because this polyatomic cation can only have one charge,