Click F12 or Ctrl + Shift + I to open the DevTools
Navigate to the Network tab
Enter method:PUT in Filter and select Fetch/XHR
Go back to Discord and react the emoji you want on the message
Now you should see a new row show up in Network tab with name %40me?location=Message or %40me
Click on it and Copy the Request URL
Done. that's it!
DevTools will only start recording HTTP requests once it is open, so you must open DevTools first before clicking the react; otherwise, the request won't show up.