How to request demand manually from a consumer to a producer_consumer?

I am trying to get a Genstage pipeline that processes asynchronous events working, but am having trouble wrapping my head around it.

I have a producer that uses a partition dispatcher to send to two different partitions. The partitions are producer-consumers that then get demand from consumers. Since the events are handled asynchronously in the consumer, I have to request demand manually in the consumer; however, it does not seem that I can implement the handle_demand callback in the producer_consumer.

Is there a conventional way I can request demand manually from a consumer to a producer_consumer?