In[]:=
V[x_,y_]:=Piecewise[{{5,x<-10&&x>10&&y<-8&&y>8},{0,x≥-10&&x≤10&&y≤8&&y≥-8}}]
In[]:=
Plot3D[V[x,y],{x,-15,15},{y,-15,15}]
Out[]=