RMB on the shell mesh item in the Simulation Navigator and select "Edit Mesh Associated Data"
Set the Thickness Source to "Field". In Specify Field, select "Formula Constructor"
On the Formula Field dialog, set the Independent Domain to "Cartesian". By default this is the Global CSYS. You can expand the Spatial Map group and select a different coordinate system to reference if needed.
You will see the independent variables in the bottom group on the dialog. Double click to add the variable to the expression in the correct syntax. You also have to correct the units. X and Y each carry a length unit. Multiplying them together gives length squared. The result of the expression must be a length unit since it is being used to define thickness. The easiest way to correct the units in this case is to make the 10000 factor a length unit as well. Do this by specifying [mm] after it. Your final expression will be:
Click the green checkmark to accept the expression edit.
OK all of the dialogs.
You can turn on the element thickness display or contour plot the thicknesses to verify the correct application