You need to use the Studio One 3 bundled with Miku. It is made specifically to allow Piapro Studio to work in Artist.
Any Studio One Artist can’t use 3rd party VSTs, so if you want to use SO4, you’ll need to buy the add-on that allows VSTs. Otherwise, use the code given to you with Miku, register it on My Presonus, and download the Studio One 3 with her that will say Piapro Studio Edition/OEM
Edit: just to explain more, Studio One Artist DOES NOT work with any VST, including Piapro Studio, normally. The Piapro Studio editions say that they are when you open the software, it will have Piapro Studio on the homepage, letting you know that is the Artist version that can use it. You do not need to manually add it in if you are using this version of Studio One, it will show up on its own. However, if you are using a Studio One not bundled with Miku, as in it was not the product you had a code to register on My Presonus with and get the software, then you will need to buy the add-on.
Edit 2: in
this link you can see the Piapro Studio logo on the homepage/start page of Studio One that I was referring to. If your Studio One does not look like that, you will need the add-on.