One need to perform below mentioned steps to assign application to the website:
1. Open IIS Manager -> Web Sites and right-click on the website that one need to assign an application pool, and then click Properties.
2. Click the Virtual Directory, Directory, or Home Directory tab.
3. If assigning a directory or virtual directory, verify that Application name is filled in. If the Application name box is not filled in, click Create, and then type a name.
4. In the Application pool list box, click the name of the application pool to which you want to assign the Web site.