Upon first visiting today, you may be greeted with a pop-up announcing the change.

Afterward, you can find a button to trigger the ad in the upper left corner, next to the bookmark icons.

This should display an embedded ad on the page. The functionality currently seems to be a bit finnicky. Not all players are having success with launching an ad, and it's not clear what the issue(s) may be; you might have success trying with a different browser or device.

After watching the ad for a sufficient amount of time, you will be provided with a reward. (Let us know what you get in the comments!)

If you attempt to launch the ad with an adblocker enabled, you will not be able to participate.

The activity appears to be available once daily, and seems to only be presented to non-Premium accounts.
I wonder if Premium accounts just get the 5k daily without watching the ads?