Jio is offering 10 GB Free addon to its customers. There is no official news or announcements about the offer, but many of the customers have already got the free 10 GB add-on pack [including me]. Some users say that, if give a missed call to 1299 from your Jio number, then the 10 GB add-on pack will be added to the account. But for some users, the add-on pack is added automatically.
So, to get the 10 GB add-on, Open My Jio app and go to My plan page, if the pack is added, then it will be visible below your current plan as shown in the screenshot.