Enable a currency for the firm.
firm_currency row for the given ISO-4217 code. The server
uppercases the input and validates against the static catalog:
400 invalid_code — code is not a three-letter alphabetic string.400 unsupported_currency — code is not present in static_currency.created: false.Use your API key (sk_live_xxx or sk_test_xxx)
ISO-4217 three-letter code (uppercased server-side).
"EUR"