put
https://gateway.stage.bill.com/connect/v3/spend/transactions//custom-fields
Update details about custom fields and their values applied to an existing transaction.
You cannot update custom fields in a locked transaction. When a transaction is locked, isLocked is set as true. You can get information about your transactions with GET /v3/spend/transactions or `GET /v3/spend/transactions/{transactionId}.
See Lock or unlock custom fields for a transaction in the BILL Help Center for more information.