Hey there, there are a few of options for you to consider. First, I list all rewards-eligible streams in my streaming rewards guide thread that is pinned on the general discussion forum. So if you have a calendar app on your smartphone or something, you can plug in those dates.
As XERDUNE suggested, if you have Twitter, follow and turn on mobile notifications to my unofficial Twitter feed @DropsWatch. I schedule a tweet for whenever a live-stream goes live making it a great way to be alerted to when these events go live.
Finally, you can turn on mobile notifications for the official Overwatch League and Overwatch Contenders YouTube channels, however, you may be alerted to non-reward eligible live streams or videos from those channels.
The next Contenders eligible broadcasts are this Saturday and Sunday starting at 11 AM PDT with the Playoff rounds of Overwatch Collegiate Championship (which are now rewards eligible).