Summary
The 'GET alert' API call in Opsgenie supports alert ID, tiny ID and Alias as valid identifiers. However, JSM-Operation API support only alert ID.

• OG API 'https://api.opsgenie.com/v2/alerts/:identifier'
• JSM API 'https://api.atlassian.com/jsm/ops/api/{cloudId}/v1/alerts/{id}'
• Compass GET Alert API

This also applies to the other POST API calls.

Why is it required?
We can create multiple workarounds and custom scripts using the API endpoints if the alias field is supported as an identifier in the GET call

Workaround:
The alias can be used with another API call List Alerts which has a query input parameter in which you can specify "status:open AND alias:<alias>" to find the current open alert identifier for <alias>. This would like like ?query=status%3A%20"open"%20and%20alias%3A%20"<alias value>"

We've also identified that indentifierType can be used, but is missing from the documentation:
https://api.atlassian.com/jsm/ops/integration/v2/alerts/<alias>/close?identifierType=alias