It depends on which version of Studio One you have.
If you have Studio One Prime, you cannot add plugins.
if you have Studio One Artist, you need the VST add-on first.
If you have Studio One Pro, or Studio One Artist with the VST add-on, follow this guide: https://support.presonus.com/hc/en-us/articles/360003455072-How-can-I-get-my-3rd-party-plug-ins-to-show-up-in-Studio-One-