Creates a new external identifier for the application or updates an existing one
if the key already exists.

Business Context:
This endpoint supports integration workflows by allowing partners to establish
and maintain cross-references between their systems and Elios Fund applications.

Update Behavior:

• If an external identifier with the same key exists, it will be updated
• If the key is new, a new external identifier will be created
• This allows for idempotent operations and easy data synchronization

Common Integration Patterns:

• Store your CRM opportunity/lead ID when creating applications
• Link to third-party project management and workflow systems
• Connect customer portal and tracking system references
• Maintain audit trails across multiple systems
• Support multi-platform data synchronization

Security:

• Requires update:applications scope
• External identifiers are scoped to your partner account
• Values should not contain sensitive customer information