如何获取ng-options值并在值内部更改时触发函数？

I have a sample code where I need to load the ajax data, I am making a call to a thirdparty APIS, and the url has a variable parameter inside it. The variable parameter is supposed to come from the ng-options of the select drop down, but it wont work, I am getting the value of undefined for the value when url is constructed.I have a sample code where I need to load the a