As far as I know, Spotify doesn’t use RSS feeds at all.
If the podcasts you are looking for are not Spotify exclusives, you will find them simply by searching using AntennaPod’s built-in search.
If they are not exclusives, but you still can’t find them, you could contact the producers and show them this page to help them make their podcast available to more users.
In general, adding podcasts by RSS address is an advanced feature, so you will most probably only need it in very specific use cases (e.g. when adding non-public or non-indexed feeds).