Usually that system includes drip lines under the poly. It could be done otherwise but not on a raised bed. Maybe you need to snake a drip line under the poly or stick a hose under and let it run slowly.

I think the idea is to run irrigation of some kind under the plastic.

I use a porous plastic that lets the rain through, but I still have a soaker hose under it.

ok, i see. i have drip irrigation that i can extend line under plastic. thanks