To use podcastfilter you need to supply two query params (plus an optional third):
query=SOME TEXT
feed=https://example.com/somefeed.xml
negate=true
The final URL should look like this: https://podcastfilter.com/?feed=https://example.com/feed.xml&query=Hi&20Bye&negate=true
podcastfilter will then return a new RSS feed you can subscribe to that only includes items whose descriptions contain the query (or if negate is true will return a feed of items that don't match the query).