Can we use CallBack of a subsystem when implementing a S-function via the legacy code tool ?

I would like to develop a S Function block on simulink that took two constants, and thanks to my extern C++ code, add or substract these values, according to a boolean parameter that the user can change to execute one or the other operation, and display the result. I have generated C++ Mex S-function via the legacy Code Tool. And it works ! My question is, how can I change or use this boolean parameter into a mask interface, by using a checkbox tool or radio button to allow the user to choose between adding or substracting these inputs ? Knowing that, because of the legacy Code Tool, i don’t have access to my C Mex file. Maybe I should totally change my method..

Thank you very much for your answer !