usually the task is part of the interview - they might (for instance) ask you how you would approach a new fundraising initiative, how you would engage the public, how you'd "sell" the fundraising.
if you're administering rather than being the fundraising manager, then you might have to do things like count collection boxes, maintain records of donations (gifts or cash), etc. Don't forget that if donors are taxpayers, the charity can claim back the tax if the person fills in the Gift Aid form.
Good luck!