If the video is playing on YouTube then there is an option bottom right corner to cast to YouTube channel on roku.
Otherwise: casting from browser only works by default with Chromecast. There are some chrome browser extensions that claim to cast to some roku devices.
Some laptops (windows)/phones-tablets (android) will screen mirror (from OS, not browser) to Roku (will need to use roku remote to accept the mirror connection). May need enabling in roku settings.
There are roku channels which claim to help but you should not need them usually, often need an app or browser extension on "sending device" as well
Apple devices will screen mirror to more recent roku (look in setting menu for airplay).
Personally I find a cheap HDMI mirroring (miracast/WiFi direct) dongle in a different HDMI port more reliable and effective than roku streaming.
Not a Roku employee