How to use pushdown predicates in Presto connector

I would like to extend Presto's example-http connector to add the relevant predicates to the REST API call made by the connector to minimize data transfer.

I have been spending now quite a few hours looking through the documentation, online posts and the presto source code but I can't figure out which calls are involved and how to go about this.

It's being mentioned in so many places, but I cannot find any simple code samples or internal descriptions. I'm sure I'm missing something obvious here.