BetterDiscord
If you've already got BetterDiscord installed, js download the file and throw it in your plugins folder. Otherwise, you'll need to install BetterDiscord first.
Download for BetterDiscordDownload the plugin
Grab QuestAutoCompleter.plugin.js using the button above. Keep the full .plugin.js filename.
Open your plugins folder
In Discord, go to User Settings → BetterDiscord → Plugins, click Open Plugins Folder, and move the downloaded file into it.
Enable & complete
Enable QuestAutoCompleter in BetterDiscord's Plugins page. Open Quests and click the purple QC button. Right-click it for settings.