If you already have access to Spotify for Artists and want to add another artist please follow these steps:
Via the web browser:
• Log in at artists.spotify.com
• Click on your avatar
• Select Create team
Via the mobile app:
• Tap on your artist image at the top
• Select Request access to a team
You cannot request access to an existing team. So if the artist profile has already been claimed, please ask the team admin to invite you.