From left sidebar menu select Integrations and click Create Webhook button:
Create new bot and click Copy Webhook URL
Go to Rtorrent-Pro main settings:
Enable Notifications by External Scripts & Enter Discord Webhook URL
Open Rdownload share and go to the scripts directory. Find example_discord.sh and place it in any of coresponding event type directory. To make it running on every event use event.download.all
Save settings and test ...
You should be now receiving notifications by Discord.