UIXv2 / Vault API: What to pass to RunExtensionMethod?

I'm making Kanbanoo ready for UIX v2 and need to call Vault Extension methods. The new Vault API has RunExtensionMethod which needs more parameters than the old ExecuteVaultExtensionMethod. I assume that application_id is my vault app's GUID as defined in its appdef.xml? What do I pass for extension and instance?

I tried to pass empty strings for extension, instance (and input) but this results in a 500 Server Error (looking at Network tab in Chrome debugger for M-Files vNext web 24.9.14055.3).

Payload source (redacted):

{"application_id":"c17f8954-1ffb-4176-aa36-XXXXXXXXXXXX","extension":"","instance":"","method_name":"com.kanbanoo.GetXXXXXX","input":""}