Add a firm-scoped custom VAT rate.
firm_vat. If a row with the same percent
already exists, the existing row is returned with created: false — the
endpoint is idempotent by percent.
When description is omitted, the standard description from
FirmVat::getOptions() is used for matching percents (e.g. 19% - TVA);
otherwise a generic <percent>% - TVA label is generated.
percent must be between 0 and 100.vat_type = none on the parent) get 400 non_vat_payer — standard rates from the catalog still appear in the
available array returned by GET /vat-rates, but custom rows are
rejected.Use your API key (sk_live_xxx or sk_test_xxx)