we want start using external api activity in a workflow, but we find very difficult to find any service to have a test. Furthermore external API documentation is very poor and doesn't show requirements and example of the API outside Campaign.
We find this video and we will be happy to reproduce it.
Can you elaborate on what the issue is? Are you getting errors? Since you're using an external API to connect to an external service outside of Campaign there isn't really any test services that we can recommend. The assumption here is that if you have a need for the external API activity, then you have an external service to connect to and in turn have access to some way to test against that service.