Hi @kristian.wijaya1234 ,
Can you please provide more details on the service you are invoking? Is it HTTP External Endpoint or Integration Server Flow service?
If its a flow service, then the service should reside on the same Integration Server as API Gateway.
You can also refer to the article which has steps to configure Custom Extension - Custom Extension policy in API Gateway - Knowledge base - webMethods - Software AG Tech Community & Forums
Please refer the documentation link below for more details
How Do I Invoke an IS Service using a Custom Extension? (softwareag.com)
Regards,
Sahana