C++ - Instantiating derived class and using base class's constructor

I have a base class with a constructor that requires one parameter (string). Then I have a derived class which also has its' own constructor. I want to instantiate the derived class and be able to set the parameter of the base class's constructor as well.

class BaseClass {
    public:
        BaseClass (string a);
};

class DerivedClass : public BaseClass {
    public:
        DerivedClass (string b);
};

int main() {
    DerivedClass abc ("Hello");
}

I'm not sure how to set the base class constructor's parameter when calling the derived class.