If your organization is on the Australian, Canadian or European instance, you will need to change the base URL from https://api.fulcrumapp.com/api/v2/ to one of the following options:

AU: https://api.fulcrumapp-au.com/api/v2/
CA: https://api.fulcrumapp-ca.com/api/v2/
EU: https://api.fulcrumapp-eu.com/api/v2/

This option is available in this API explorer by clicking "Base Url" and choosing the correct option for your instance configuration.

For language specific SDKs, you will need to override the base URL in the client initialization as appropriate.