When IQVIA posts a new position on its great careers page, it also advertises positions through many different industry and career-based groups and organizations. They ask this question in part to hear firsthand where their best candidates are being sourced from so they can continue to place resources into areas that work in the future. But on top of this, they're also looking to hear if you have any inside connections to the organization or a deeper interest in coming to work there by asking this. Be open and honest with how you found out about the opportunity and reiterate how you feel this is the perfect job for you.