Configure Mock before InjectMocks

I have code that looks like this:

@ExtendWith(MockitoExtension.class)
class MyTest{
    @Mock
    private ClassA classA;

    @InjectMocks
    private ClassB classB;

    //Tests omitted
}

@Service
class ClassB{

    private final String stuff;

    ClassB(ClassA classA){
         this.stuff = classA.getStuff();
    }
    // Code omitted
}

I need to mock the getStuff method. How do I mock the method before the injectmocks annotation is injecting my mock of ClassA? This is really easy without annotations but I would like to do it with annotations if possible.