Just click the Cast button on the browser. If it is not there click the 3 dot Settings and choose Cast. Works fine for YouTube on my son's Chromebook.
TheTick Roku 3 for Pokémon TV / YouTube / Animal Planet GO / HBO GO. Chromecast for most other media because Roku Mirroring sucks , how many clicks in Wndows 10 (about a dozen) to Miracst on Roku vs 2 with Chromecast. Verizon FIOS