Use this template in exactly the same way as {{it-adj}}
. The only difference is the category that gets generated is Category:Italian superlative adjectives. However, since all superlatives of Italian adjectives are regular, you should not need to use any parameters.
For example, for bellissimo, use:
{{it-adj-sup}}
which produces
bellissimo (feminine bellissima, masculine plural bellissimi, feminine plural bellissime)