The Windows Experience Index (WEI) is a benchmarking system which provides performance scores for your PC's processor, RAM, hard drive, 2D and 3D graphics. The interface for WEI was dropped in Windows 8.1, but the command line tool which ran the tests (winsat.exe) is still present, and ChrisPC Win Experience Index uses this to calculate and display your PC's ratings.
The benchmarking process works just as it did with the original tool. Close any other running applications, launch ChrisPC Win Experience Index, and click "Rate this computer". The program will then carry out its various tests while you wait. Once it's finished, you'll see separate scores for your CPU, RAM, graphics and hard drive, with the lowest highlighted as your "base score"; if someone's asked for your system WEI, then this is probably the figure they want.
If any of these scores appears significantly lower than you expected, it could mean some other process was also grabbing system resources while ChrisPC Win Experience Index was running. Make sure there's no background program running some heavy duty task (a virus scan, say), then click "Re-run the assessment" to try again.
If there are still any issues then you can use the Windows tool winsat.exe to create a report manually (launch an elevated command prompt and type winsat /? for your options). ChrisPC Win Experience Index uses winsat.exe itself, though, so this shouldn't make any difference: the program will display the same scores that you'd have seen with Windows 10.
PLEASE NOTE: the setup program will by default install adware along with its own code. To avoid this, pay close attention during setup, and choose the "Decline" option or clear the relevant checkboxes as necessary.
A simple but effective way to restore the Windows Experience Index in Windows 10.