You can add them as favorites within the TuneIn section of the Echo App or within the iHeart Radio section. Both work the same way--so where it says "TuneIn" below, read "iHeart Radio" if that's the one you're working with.
To do that, select TuneIn from the sidebar menu within the app. Search for the station (I just searched for German radio and found Antenne Bayern, as well as a bunch of others.) It should switch to that and start playing. :D You'll see the "play bar" at the bottom of the screen, below the list of search results. Tap on the TuneIn or Station icon. (Some stations have their own icon.)
Now the station play bar will fill the right side of the app. On the right side, you'll see "Queue" and "History" and the name of the current song underneath.
To the right of the song that's playing will be a little gray down arrow that is hard to see. Tap on that. When you do, you should see the option "Favorite Station" in gray. Tap on that. It should turn red.
Now, go to the Home Screen and tap on TuneIn again. Scroll the right side of the app where it says Browse, Local radio, Trending, etc, to see the bottom of the list.
Under Favorites, you should see your station and you can play them from there. In the future, you'll only have to do the parts under the *** to play favorites.While this is talking about a smart device app, I found that the web interface worked just the same.