How Hathaway ISP shows renewal reminder in non secure website?

I am using Hathaway ISP's broadband plan in India. I have a prepaid plan which needs to get recharged every month. Few more days are left for the plan to get expired and I observed the below screenshot.

This popup comes only when I visit any non-secure website (without https in their website's URL)

How do they(ISP) guys do this?

Thanks in advance.