Do you have the 3D Model as OBJ or FBX or anything similar?
If yes - import it in a 3D Modelling Software, move to the Center as you need and the Center is the Pivot Point
Export from 3D Modelling Software and import to UE4 back - now you have your mesh as intended
Edit: Or wait… 
you mean the whole BP pivot point? it’s always the Center where the Root is
Why do you want the Box as root?
You can f.ex. use the Plane and its collision (if it doesn’t have any you can generate it in UE)
Or do you have any particular reason for that setup where the box is the Root Component?
But if you just rotate the Plane then I assume the Plane itself has Pivot Point underneath so you probably need my 1st idea solution 
Also be aware that if the Box is not moved and it’s the Root - it’s always gonna stay in Place
Is that intended? Actually you could use that as the landing point afterwards as an example 