post https://api.getsilverfin.com/api/v4/f//companies//periods//reconciliations//custom
You can either submit one custom property by posting a namespace, key and
value, or you can submit a bunch by submitting an array in properties
containing objects with namespace, key and value.
You can destroy an item by setting the value to nil when there are no
documents attached.
Required scope(s): financials:write