Hello Peter, I looked again at the link and video yu suggested ( but the thing is I don't want any other bodies to be imported (like extra sketches to control the curve) into WB from solidworks CAD and I'm guessing if Direct Optimization would then act on the extra sketch to fix the curve at each desing point? I don't think so, as it works only on what is imported in Geometry of Workbench... What would you say?

By the way, the global variable way looks more appropriate and I tried to do as follows in SW

global variable "c"=0.65; global variable "a"=29 and equation given by 

y=0.2/(1+"a"*(x+"c") with x1=-0.65 (-"c" wouldn't work) and x2=0

I'm not sure if I should give dimension to Lc in such case as it would be contrained? I give colinear constrain as you suggested and imported everything into Ansys Design Modeler but it reads 65 for "c" instead of "0.65", which is quite strange because all other parameters are imported correctly, and they are decimal as well (SW works with dots whereas DM works with commas). Any suggestion about that? 

Maybe if I could be able to parametrize the curve in SW with a and c and pass these two as parameters into Design Modeller, then direct optimization would be able to update the curve when giving a,c as input parameters?

I thank you again for your help