How do I grant permissions to manage test plans in VSO?

I am unsure whether this is the correct stackexchange site to ask this question, however it is about software tools commonly used by programmers (sort of).

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I am collection administrator of 100s of .net projects that I have recently migrated from an on-premise TFS to VSO. I am receiving requests from developers to grant them permission to create/manage test plans.

Now, these developers are members of the "Developer" and "Contributor" groups and as such have the following permissions:

But even so, they are unable to create test plans (using Microsoft Test Manager 2013). I have searched throughout the VSO control panel and I cannot find out why my developers don't have permission to create/manage test plans.

Even the documention provided by Microsoft seems to fall short when it comes to managing permissions in VSO.

Any help is much appreciated. How do I grant my developers these permissions?

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Edit: I should add that I myself am able to create test plans. I have compared my permissions in a given project with one of the developers and they are pretty much equal (with some unrelated exceptions like deleting projects).

You can control the "Manage test plan" permission on area node level.
Go to area part, right click on the area your test plans belong to and then click on "Security"

Now you can grant rights for managing test plans: