Just taking an example from their documentation, you would need to manually add a new header called Authorization and then add Bearer <token> as the value.
curl -X GET "https://ob.nordigen.com/api/v2/institutions/?country=gb"
-H "accept: application/json"
-H "Authorization: Bearer ACCESS_TOKEN"
You can use use the Bearer Token helper in the Authorization tab of the request, this will auto-create the header once you add the value.
