C++ how to make a prototype of this kind of constructor?

I am honestly not sure how to google this even, and as my attempts have failed to do so, can you tell me how to write a prototype of constructor so that I can use it this way?

// MyClass.h
class Object;
class MyClass {
    Object a;
    Object b;
    std::string c;

    public:
    MyClass(int, int, std::string&); // I do not know how to declare this properly
};
// so that I can write this:
MyClass::MyClass(int a, int b, std::string& c = "uninstantialised") : a(a), b(b) {
    this->c = c;
}
// so that when I call the constructor like this:
Object a();
Object b();
MyClass mc(a, b);
// it doesn't produce an error when std::string argument is not specified.

Thanks!