As the related announcement is locked, I have to start a new topic about it. I just tried to use this feature, but could not figure out for a while why I could not find any place in Discord to somehow add WACUP to my "game activities", because that is what you have to do as well if you want to show your currently playing song on a Discord server in your profile.
Long story short: the tab "game activity" in your settings only exists in their desktop version, not in the browser version which I normally use. In that tab you can open a dropdown list of active Windows apps in your taskbar (not try to type WACUP), so you can just pick WACUP from there if it is already playing something.
Furthermore I had to disable and enable the related switch there and also click on the Overlay button above the switch, then it would finally work, also in the browser. Closing the desktop app ended the display of the current song in my Discord profile, although I still had it open in the browser.
When it worked, it updated the new song immediately for me, no lag. Maybe this also depends on the number of server members. I tried it on the Last.fm Discord server (about 2k members, several hundred online), as WACUP still lacks a native scrobbling plugin. ;-)