Hey, @meowsiclez!
This is a forum for third party developers building on the Twitch platform. Your question is more general around front-end development. I would recommend StackOverflow for questions of that nature.
Having said that, my guess is that it is simply an HTML element with ::after pseudo-element that is animated using CSS animation. The pseudo-element will be absolutely positioned to make it overlap the input box. That’s how I would implement it. 
Edit: I was interested in how to accomplish this, so here is a JSFiddle: https://jsfiddle.net/ztd76L63/