set a system prompt/message for queryengine

i cannot for the life of me figure out how to set a system prompt while using index.asQueryEngine()

could someone point me in the right direction? this has to be possible, otherwise how could anyone use llamaindexts for RAG applications?

here's a related discord thread: https://discord.com/channels/1059199217496772688/1133167189860565033/1214393825473077248