Can Jira Service Management portal-only customers (external customers without a full Atlassian/product license) authenticate into a third-party web app via Atlassian OAuth 2.0 (3LO) — i.e., can we reuse their existing JSM identity to sign them into our own app? Or is 3LO limited to full Atlassian accounts? If it’s not supported for portal-only customers, what’s the recommended approach for authenticating them into an external app?
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No, OAuth 2.0 (3LO) is not supported for JSM portal-only customers. Portal-only accounts exist in a separate identity system from Atlassian accounts.