In Optimization Design, how to select one part's parameters in assembly situation?

I am doing an assembly static analysis. In Optimization Design module, I want to take one part's parameters as variables, as show in Picture 1. But when select variables as shown in Picture 2, only  the parameters describing assembly are available as shown in Picture 3. How to reach my goal?

                                             Picture 1

                                                  Picture 2

                                                              Picture 3