When trying to determine what to do, you first need to understand the type of threat that you actually encountered, which is what you appear to be doing here.
Breaking down that threat name Trojan:HTML/Phish!MSR a bit further, you'll see that the Trojan portion may not be as bad as it sounds.
The HTML portion indicates this item was found in an html page, which makes sense since you mentioned you were browsing the Twitch website.
The Phish portion indicates this was detecting a Phishing attempt, so likely just a web page that contained a request for information like username, password or other personal information.
The "!MSR" portion is a code I don't recognize, though the exclamation point implies this was an AI (Artificial Intelligence) machine based, automated detection and these can be prone to false positive results, since no automated system is perfect.
So in truth, this was likely nothing more than either a popup or something else on the page being browsed at Twitch, possibly even a rogue advertisement that displayed a popup or other page containing a request for information that the AI machine based, detection systems decided was suspicious due to certain attributes.
In other words, the detection, though titled as a "Trojan", was likely nothing more than a transient web page of some sort, that since these go away when you close them, won't likely be a problem unless you encounter that same or a similar web page again.
So there's really nothing you need to do other than be aware that if this detection occurs again, you should be wary of any requests for you to fill in any blanks with personal information on that page.
Rob