Please clarify it this should be done in a view on the acutual website, or in edit mode for editors (dojo/dijit)?
Maybe something like this http://world.episerver.com/Blogs/Linus-Ekstrom/Dates/2013/12/SingleMultiple-selection-in-EPiServer-75/? Not multiselect in a dropdown, but as checkboxes. Multiselect in a dropdown is not that common.
I am working Episerver CMS 9, I want to filter sub categories by parent category with name or Id( anything is fine). The result need to display as dropdown source with multi selection.Can anyone help us on this?