Problem with Web API and integration

Hello,

I'm creating a Web API for the first time, and I'm facing issues, perhaps due to not knowing how to test it.

I have the Web API set up, and when I click the button to test it, it returns the value correctly:

However, when trying to do it from an integration or using Postman, it doesn't return the response, but rather the following:

What am I doing wrong?

Thank you very much.