The promotion starts at 12:00 am PDT (3:00 am EDT) on Friday, September 3rd, and runs for four full days, ending at the same time on Tuesday, September 7th. Once you plug your EV in, you'll be instructed on what to do in order to receive the free session. The email states: "All you need to do is plug-in and follow the prompts. No promotion code is needed." https://insideevs.com/news/530361/electrify-america-free-charging-offer/
That link includes a good explanation of the reason why in some states Electrify America still charges by time.