As it stands, the API can be used to add collection items, including populating reference and multi-reference fields. However, these reference fields cannot be created via API - you need to create them manually and then use the API to add content.
It would be great if you could:
Create reference fields via API
Create multi-reference fields via API
Sam van den Nieuwenhof
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This is absolutely needed. You can retrieve the field data when getting collections but not create them