To do shape optimization with SW and WB, you want SW to have a global variable with a DS_ prefix that WB uses to look for the few parameters from the complete set in SW to use. In this case "DS_c" would be used. WB sends a new value for DS_c to SW, which updates the geometry and sends a new solid body back to WB.
The properties of the Geometry cell in WB limits what is being imported. In your case it would be set to Solids, so no sketches will be imported.
The first method I tried (without a second sketch) in SW did not update when I changed the value and Rebuilt the model. Try it with the "a" parameter since that is very easy to visually check as you go from a=19 to a=49 that there is a large change in curvature. But when I edited the value of the Global Variable in the Equations... table and clicked Rebuild, there was no change in shape. That is why I recommend you try to follow the more complicated second method that requires a second sketch with a dimension on it, and manually build the Parent-Child relationship between the two sketches that triggers the update to happen.
ANSYS does not respect the regional setting that lets you use a comma as the decimal separator. It was written assuming that the period is the decimal separator. Change your regional settings to use period as the decimal separator. That will make "c" come in as 0.65 instead of 65.
You said that in SW, you couldn't type -"c" in the X1 field. You can't just type -0.65 if you want that to be a parameter. The work around is to create a Global Variable "e" that is set equal to -c in the Equations... table. Then you can type "e" in the field for X1. Or you could just set "c" to a negative value and change the sign in front of "c" in the curve equation.
You can't overconstrain your sketch, nor do you want it under-contrained. You have to figure out what works and make sure that when you type new values in the Tools > Equations... table in SW, that the shape changes appropriately.