to draw an equation based curve in HFSS, from the Draw menu choose equation based curve and in the opened dialog enter the equation you want.
Here is an example of drawing the exponential curve y=A*exp(B*x)+C in the interval [0,10] :
X(_t) = _t
X(_t) = A*exp(B*_t)+C
Z(_t) = 0
Start_t =0
End_t =10
after drawing all edges select all of them and unite them,then to create a surface right click on it and from the popup menu choose edit->surface->cover lines