You're using the Portuguese transliteration of the eñe. And if you were doing it with an Italian or French transliteration, you could write "agno."

(That said, it's one that's so common it's worth learning the keyboard shortcut for. On a Mac it's "opt-n, n".)

        setxkbmap us -variant intl -option compose:rwin]

Press altgr or right win key + [aeiou] or n.