retail_portfolio_id
to the portfolio available when making POST calls to Create Order on spot products. You can retrieve portfolio IDs from the List Portfolios API and then see which accounts you were granted access to from the List Accounts API. If all accounts belong to the same portfolio ID then you can use it for all OAuth based Create Order calls or you can present portfolio selection to your users or deduce the correct portfolio based on the accounts available.
Example request body on Create Order API for OAuth connections only: