In the User interface (UI) tabpage you will see the UIRef element that is used in the script.
At this point the installer can build an will work but it has no user interface.
If you want to change for example the software location of select or deselect features during the installation. To change that you can click the button on the UIRef Header to create the element and set the Id to "InstallDir"
WixPie will ask you for the Target directory that you can select from the dropdown.
For this tutorial we will select the INSTALLFOLDER
For this to work you need to add the reference WixUIExtension to the Visual Studio project, in visual studio you can select add reference in the menu.
Now you can press CTRL + S or go to the File > Save menu and go to Visual Studio and build the installer project the result will be something like the image below.