It is not possible to bookmark an artist on the Android. Instead, you can “Like” the artist or song.
First, go to the Backstage area. To get there, touch the icon in the upper right with four arrows pointing in. Then under “About” the artist select “Show More.” There you will find the option to Like the artist or song. Note that Likes are different from Thumbs and won’t have any affect on your station.
To see your list of Likes, you will need to sign in to your account on a computer.