This will depend on the carrier for this shipment. In some cases, carriers will send the necessary customs forms for an international shipment back to your ShipStation account along with the label so you can print them out. For some carriers, customs forms are submitted electronically by ShipStation, and so do not need to be printed at all.
In a few cases, as with Royal Mail, customs forms must be created outside of ShipStation.
For details on your specific carrier and how customs forms are managed, review your carrier's integration page in our Integration Help guide.