Go to your Fountain library tab

Click the 3 dots

Click "Add Private RSS Feed

Paste the nostr:npub196qvw7utjs0cnztlg4aww98ekql9svm6c4wlv6sug70nzz0uujxsprafme RSS feed there

This one form the page and not the url. They do look the same and you want the one that has .xml at the end like the one below.

https://castr.me/npub1n00yy9y3704drtpph5wszen64w287nquftkcwcjv7gnnkpk2q54s73000n/rss.xml