The opposite (antonym) of antonym is synonym. An antonym is a word that means the opposite. A synonym is a word that means the same thing, or nearly the same thing, for a given context.
Some words do not have direct opposites [antonyms]. Antonyms do not usually substitute for phrases. The phrase "you are" has no antonym. A contrasting version of "you are" is "you are not".
Some words do not have direct opposites [antonyms]. The word "will" as a noun has no antonym. If "will" is used as synonyms for like, please, or prefer (and other similar words), then the antonym for...