Thank you for reaching out to the Ansys learning forum and your patience is much appreciated.
Do you know that there is a "draw torus" command in the draw ribbon area of the Ansys Electronics Desktop? This can simplify your model creation.
If you still want to use an equation based model geometry then please go through "drawing an equation-based curve" section of the HFSS help pdf document which can be accessible from the help menu. Kindly note that default units for equation based curves are meters and unit conversion is not properly supported.