MVC 4根据DropDownListFor选择更改字段

I have a form that has a dropdownlist that is made up of different course names and a textbox that will contain the number of the next course section based on the dropdown selection and its highest existing section number in the database. After a selection is made the javascript will get called, then call my controller method to figure out which section number it should be, and then fill the textbox. I have a form that has a dropdownlist that is m