Bought Program on RPGWeb, how do i register it on Steam?

[allyneko223]

I want a DLC for MV that's being sold on steam, SAKAN Tileset Editor, but Steam isn't allowing me to purchase it since I don't have MV in my library.
Does anyone know a way for me to register MV onto my steam account so I can get this DLC without spending another $70?

 

[Archeia]

Use your RMMV Key on Steam.