Thanks! It worked!
After your guidance I was able to create the new account and access Vikunja desktop!
curl localhost:3456/api/v1/info
{"version":"v0.19.0","frontend_url":"","motd":"","link_sharing_enabled":true,"max_file_size":"20MB","registration_enabled":true,"available_migrators":["vikunja-file"],"task_attachments_enabled":true,"enabled_background_providers":["upload"],"totp_enabled":true,"legal":{"imprint_url":"","privacy_policy_url":""},"caldav_enabled":true,"auth":{"local":{"enabled":true},"openid_connect":{"enabled":false,"redirect_url":"\u003cfrontend url\u003e","providers":null}},"email_reminders_enabled":true,"user_deletion_enabled":true,"task_comments_enabled":true}