Hello Kwinten Volckaert,
Thank you for pointing out this issue. Unfortunately, providing a selector for metadata fields is only available for the default ones that we have included in the modules. Since this functionality may be needed for different types of metadata and in different places, we will consider providing a general way to create selectors for one of the upcoming releases. We're sorry for the inconvenience.
Your Telerik points have been updated!
the Telerik team