Hereon is defined as upon, on or immediately following.
(adverb)An example of hereon used as an adverb is in the sentence, "He will pay off his credit card bill, hereon receiving his paycheck," which means "He will pay off his credit card bill, upon receiving his paycheck."
See hereon in Webster's New World College Dictionary
adverb
See hereon in American Heritage Dictionary 4
adverb