In case this button doesn't activate anything for you, try looking into browser's menu for 'Add to Home Screen' option.
Try it out, it works best on Android and Windows devices, when installed using Chrome browser. It's a nice app, it will never require any updates, as it will automatically update when our Youtube search and can convert videos to mp3, on top of normal video downloads, of course. It's full of features as much as the website is.
It's light-weight and takes very little space on your device.
