Sorry that I have to "answer" you question when I should actually "comment" to ask if you're okay with using a python module instead of a REST API (not enough reputation).
Anyway, I love playing with stock data, here is my method for getting it for free in python and exporting it to a csv.
pip install yfinance
import yfinance as yf
The official website of the index is
If you hover with your mouse over Export above the graph, then you can recover a link, with which I can download the last ten years' worth of closing prices up until yesterday in XLS format (I've removed the hostIdentifier part):