Are you an employer wishing to post a non-European Union citizen employee to Flanders/Belgium for more than 90 days?
In that case you need to apply for a single permit that will be processed by the competent region (Flanders/ Wallonia/Brussels Capital Region). Once your application has been accepted, your employee will receive a single permit permitting him or her to reside in Belgium for work for more than 90 days.
All you need to know on the following website:
https://www.international.socialsecurity.be/working_in_belgium/nl/gecombineerde-vergunning.html