WordNet records no synonym and no antonym for behalf. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
as the agent of or on someone's part (usually expressed as `on behalf of' rather than `in behalf of')
“the guardian signed the contract on behalf of the minor child”
for someone's benefit (usually expressed as `in behalf' rather than `on behalf' and usually with a possessive)
“in your behalf”
Synonyms, antonyms and the broader and narrower relations for behalf come from Princeton University’s WordNet. Full sources and licences.