All you have to do is click the add button on the pop-up after opening a magnet link. The popup window in Dekuge may sometimes be blank at first, but you can click add before it populates and it still works.

IDK if there's a way to get it to skip that part and just start downloading after opening the link, but if there is and someone else knows I'd like to know, too.

When you attempt to open a link in your browser it calls the executable you specify with the link URL as its first argument. Glancing at the deluge torrent source, it seems to want a different command line than the one the web browser provides. You can fix this using a small wrapping script.

For macOS, there's a separate application you need to get magnet links to work. I think this is it but am unable to check currently: https://github.com/Lord-Kamina/Deluge-Magnet-Handler

Browser extension that connects to the deluge webui. But my deluge is running on unraid on my Nas. Would probably work for the desktop deluge, if you can setup the webui.

Check the app settings and make sure it has permission to open links

I'm running deluge in a docker container on unraid and I have this same issue in Firefox but not in Chrome. I also have authentication problems in qBittorrent in Firefox that don't happen in Chrome. It's literally the only thing I use Chrome for unfortunately.

Before I had it all automated, I used to use a browser extension which would send them to the web ui without having to actually open deluge at all. I think it was called deluge siphon.