Step function integration with internal API using http task

Hello, Step functions now provides direct integration with 3rd party APIs instead of using lambdas for API invocation. I was able to integrate external API with Step function by following this article. https://docs.aws.amazon.com/step-functions/latest/dg/connect-third-party-apis.html.

But this is failing if integrated with internal API with an error like 'Unable to resolve domain 'internal api url here' to IpV4 address.

Does this mean internal APIs that are not publicly exposed can't be used? I wouldn't want to make this API public just to get the integration working or to set up an API gateway. Is there any other way than using lambda?

Thank you!