PhD candidates can be employed by the faculty Educational Sciences but the faculty also offers aspirant PhD candidates who are employed elsewhere the opportunity to get their PhD. These so called external PhD candidates are not a part of the faculty Educational Sciences but their project is supervised by one of the professors.
Your research question for a PhD should match one of the areas of interest of the research programme of the faculty Educational Sciences.
Check also the list of earlier PhD-research projects or current research projects of the faculty Educational Sciences.