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You will find this bar in the upper half of your screen, labelled in white. On the mobile app, tap on the three horizontal lines in the top-left corner of the screen to reveal a menu containing the search function. Tap on "Search" to activate it. You will see matching results appear in the menu that drops down from this search bar. You will be brought to that artist's Spotify page. You'll now receive updates in your music stream whenever that artist releases new music on Spotify.
Tap/click on the search bar. Type in the name of an artist you'd like to follow. Tap or click on the artist that matches your search. Tap or click on "Follow".