How to drag my armory from GUI to Terrain?

I am making some kind of RTS game, to mimic the visual effect like SC.

When the gamer click the armory UI, the transparent model is appearing, then gamer can drag it to terrain to build this building wherever they want.

So far, I can place the building on the terrain by click, but I hope gamer can drag it to the scene.

Thus, what kind of function I can use in Unity 3D? please give me some advices or instructiions, thanks for your help.

It depends on how your armory model was built. The easiest thing would be instantiate the transparent version of your armory model once you click on the gui item. Then if the pivot point of your model is a the bottom you could do raycasts from your mousePosition to the ground and set the pivot point to the intersection.