Hereonto is likely misspelled.
Hereonto has no English definition.
As an adverb hereon
is on this place.
hereon
English
Adverb
(-)
On this place
To this place
On this subject or basis
hereupon
References
* OED 2nd edition 1989
hereonto
Not English
Hereonto has no English definition. It may be misspelled.