How change text of textobject based on user's selection?

How to add a label at page header and change text based on user's selection from a asp.net page?

For example, I created a CR as Order.rpt. I added a textobject into page header. From one asp.net page Order.aspx, user can select order city by selecting a dropdown list, such as New York, London. I want the city name display on the CR on run time.