Creates a new external identifier for the design 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 designs.

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 project/design ID when creating designs
• Link to third-party design tools and their project references
• Connect workflow management system task IDs
• Maintain audit trails across multiple systems
• Support customer portal cross-references

Security:

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