You don't need any external preamp. The preamps on board are very good, and you can control them in Studio One. There is a lot to learn, so here are two manuals, which provide basic information. You find the manuals on the download page of your mixer.
* Capture3_ReferenceManual_EN_25032019
* Studio_One_Integration_and_Quick_Start_for_StudioLive_Mixers_Reference_Manual_V2-EN_25032019
Have a look at the many videos PreSonus has published about Studio One. There have been 6 videos added just this week dealing with recording and DAW mode.