Remote drop seatpost cable route
It partially depends on where the cable comes out of the post. If it's from the top you can route the cable across the bottom of the top tube and up to the handle bar or down the seat tube and up the down tube. On the other hand, if the cable comes out of the bottom of the post, you can try to go out the bottom of the seat tube and up the down tube.

What is also becoming more common on frames without designated routing for dropper posts is to run a 1x setup and re-use the routing for the front derailleur cable for the dropper cable.