They want you to work in advance and for you to ask them later with thorough explanation for justifying a raise. They'll decide later (end of the year) performance review or salary review.
I applied for an extra role at work last year. They were reluctant to give it to me "because I hadn't already been doing it" (for free)...shows that I'm not keen. Why the hell would I have done this before when I had any time or resource for it? They didn't have any other applicants so they gave it to me anyway.....but WTF