Right now, you can earn free Overwatch Contenders skins by watching the live broadcast from the Path to Pro website. To kick things off, throughout the rest of October you can look to earn a cool Mercy and Symmetra skin, in the Contender's black and green, as shown in the Contender's Twitter post.
Fans who watch the stream and are eligible to receive rewards will be getting drops for both the home and away Contender's variant for each hero. Currently, to earn the Symmetra skin, fans must clock in 7 hours of watch time on the stream, with the Mercy skins requiring 15 total hours watched. At the end of the month, the clock will reset as new heroes rotate in with skins to earn.
Blizzard has even stated in a post on the Overwatch Path to Pro website that "special moments in the competitive calendar for Overwatch Contenders (e.g. Gauntlet) may feature event-specific incentives," so be sure to keep an eye out for them.