Monthly subscription on autopay?

I would like to set up a monthly subscription to my services that is on autopay which processes the last business day of the month for services to be rendered in the coming month. So the client pays in advance of the service which motivates them to schedule their allotted sessions that are part of the package and to decrease no shows as already paid. I already have this listed as unit billing but can’t figure out the prepay part and would prefer to have it on autopay