Hi Filip,
In Testiny, a defect (and a requirement as well) is a workitem.
The workitem API is not officially documented yet, but I can share it here so that you can already use it. In the future, we’d want to offer an easier way to create and retrieve workitems.
To retrieve a workitem that is linked to another entity, you’ll need to use the map parameter.
Workitems are currently not directly linked to a milestone, but to test cases, to test cases in runs, and to test runs.
- Here’s an example how to retrieve work items linked to a test case: Add Jira requirements to test cases when created via API - #6 by hanna_mueller
To retrieve workitemes from test cases in a test run, the entities would be ["workitem","testrun", "testcase"].
You can also check the API calls that are made from Testiny DevTool’s Network tab for more examples/details.
I hope that helps you to get started and let us know if you have more questions 
Best regards,
Hanna