patch https://app.moderntreasury.com/api/external_accounts/
Partial Updates
Only top-level fields that are passed will be updated. If you wish to keep a value unchanged, simply omit the information from the body of your request.
Counterparties
You may only add counterparties to external accounts which do not already have an assigned counterparty. You may not update the counterparty of an external account that has a pre-existing counterparty.