how to pass in a std::shared_ptr but do "pass by reference" job?

I have a main function that takes in a reference of a class object and try to update it via foo() and bar(), however, the bar() only allows to pass into a shared_ptr type. How can I guarantee it behaves as expected, assuming I am allowed to modify the signature of bar().

void foo(MyStruct& s) {
  modifyS(s);
}

void bar(std::shared_ptr<MyStruct> s) {
  modifySAgain(s);
}

mainFunction(MyStruct& s) {
  foo(s);
  // bar(?) how should I do here such that s would be modified by bar()
  // bar(std::make_shared<Mystruct>(std::move(s)) ?
}