exactly vs on the button

exactly is not an adjective while on the button is an adjective.

Word	Noun	Adjective	Verb	Adverb
exactly	No	No	No	Yes
on the button	No	Yes	No	Yes

As adverbs, exactly and on the button are synonyms defined as:

Other synonyms of exactly include on the dot, on the nose, precisely.

exactly (adjective)	on the button (adjective)
	being precise with regard to a prescribed or specified criterion

exactly (adverb)	on the button (adverb)
indicating exactness or preciseness	just as it should be
in a precise manner
just as it should be