POST /v3/instruments is polymorphic. Specify --type to see the full schema:

Createable types:
  bond-instrument
  fx-forward-cash-settlement-instrument
  fx-forward-physical-delivery-instrument
  index-linked-bond-instrument
  loan-instrument
  mutual-fund-instrument
  private-market-fund-commitment-based-instrument
  stock-instrument
  zero-coupon-bond-instrument

These types exist in responses but cannot be created via POST:
  fee-instrument
  refund-instrument
  other-instrument

Example: aleta api --spec POST /v3/instruments --type bond-instrument