Travellers to Barbados are required to complete the online Immigration and Customs Form which will be available 3 days prior to your arrival and is valid up to the day you are leaving. It is not mandatory that you complete this Form before you leave as you can always use the kiosks located in the arrivals hall before Immigration.
Be cautious of websites asking for payment to complete the Barbados travel form, as these are likely scams. The official Barbados travel form is free, and you can complete it at a kiosk upon arrival or online at travelform.gov.bb.