They call it spaces, so when you create a "Space" on Desktop at least, you look on the right hand corner for the three dots, next to the Share button, and click "Settings":

> Give instructions to Perplexity to customize how answers are focused and structured.

You type into that box, now every space you make can have a fully custom "pre-prompt" as I call it.

I only just started using Perplexity, they really need to rework their UI a little bit.