f[x_,y_]=1-(x^2+y^2)

1-(x^2)-(y^2)

G=Plot3D[f[x,y],{x,0,1},{y,0,1},BoxRatios->{1, 1, 1},MeshFunctions->{#3&},Mesh->5,Boxed->False,Axes->False,RegionFunction->((#1-0.5)^2+(#2-0.3)^2<1/16&),PlotStyle->Opacity[0.4]]

Ax=Graphics3D[{Arrow[{{0,0,0},{0,0,1}}],Arrow[{{0,0,0},{1,0,0}}],Arrow[{{0,0,0},{0,1,0}}],Style[{Text[x,{1.1,0,0}],Text[y,{0,1.1,0}],Text[z,{0,0,1.1}],Text["P",{0.52,0.32,0.72}]},Large]},Boxed->False,Axes->False]

DD=Plot3D[0,{x,0,1},{y,0,1},BoxRatios->{1, 1, 1},Mesh->None,Boxed->False,Axes->False,RegionFunction->((#1-0.5)^2+(#2-0.3)^2<1/16&),PlotStyle->Opacity[0.4]]

Pt=Graphics3D[{{Red,Point[{0.5,0.3,0}]},{Red,Point[{0.5,0.3,f[0.5,0.3]}]},{Red,Point[{0,0,f[0.5,0.3]}]}},Boxed->False]

L=Graphics3D[{Dashed,Line[{{0,0,0},{0.5,0.3,0},{0.5,0.3,f[0.5,0.3]},{0,0,f[0.5,0.3]}}]},Boxed->False]

Show[{G,DD,Ax,Pt,L},ViewPoint->{13,24,20}]