To import the OpenAPI definition in the NWC login to your NWC tenancy and open the Xtensions section. You can add a new custom connector there. Select the exported OpenAPI file and click on Next.

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In the Security section APIKey is preselected as authorization method. You can leave the standard values for Name and Location and click on Next.

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After that enter a name, description and choose an icon for your custom connector. Then you can publish it.

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After publishing the custom connector will be automatically available in the NWC workflow designer.