TFS 2017 power tool custom field connected to User Story

Using the power tool I was able to add a new custom field but I want that it take automatically a specific value if present.

Each test case has a "user story" associated but to associate it we have to go in "Tested User Story Section" so I'd like that in the main page appear automatically the ID of the user story.

Sorry for the confusion maybe with the image will be more understandable:

Someone can advice me of what to introduce in the Visual Studio Power Tool please? because I'm finding different example but none explain how to auto-populate with another dynamic item.

If you are asking me why it's because we want to have the "User Story ID" present in the main page so we know directly at which user story the test case is associated:

There is no way to get the User Story ID as a field. Since a single Test Case can test multiple User Stories this makes little sense anyway.

Have you tried adding Requirement based Suits to Test Professional so that you can see all of the Test Cases under a Story? From a testing perspective it makes lots of sense to look from the Story down to Tests rather than Test up to Stories.

Although my screenshots are from the Web App the features at the same in MTM. You can add a Requirement Based Test Suite that relates back to the User Story.