I have a custom field on my expenses called "Cost Centre" and I wish to be able to update the value via the API. However the documentation  https://www.zoho.com/books/api/v3/#Expenses_Update_an_Expense doesn't show attributes for these fields, and attempting a request like

JSONString= {"custom_fields": [{"label": "Cost Centre","value": "Management Delegated Budget"}}"

throws an error "JSON is not well formed." Can you give an example of what needs to be in the JSONString to do this please?