You will need to install a user script manager extension to install this script.
This script adds a button in Discord's text box that allows you to automatically add reactions to new messages in text channels for however long is specified. The entire system has been redesigned to allow multiple reactions to be added in rapid succession, and to preceding messages as well. Be warned - This script is powerful, and not endorsed by Discord in any way. Popups for Discord must be enabled for the script to work.