Create a partner fee charged to your customers, under the given account.

Scope:

• Create on the main account to set a default fee that applies to all sub-accounts.
• Create on a sub-account to set a fee that applies only to that sub-account.

Fee matching: when a transaction occurs, a sub-account fee takes priority over the main-account default, and the most targeted fee wins.

external_id: required valid UUID in the request body, unique to this create request. Reusing an existing value returns a conflict. If a previous create timed out, look it up via the list endpoint with the external_id query parameter instead of replaying the create request.