Custom list definition with folder creation

Good morning!

I am trying deploy a custom list definition from a content type (which inherits from item content type) with the option FolderCreation="TRUE" from Visual Studio 2010 to a new Sharepoint 2010 site, but when I push the "New Folder" button in the list, the new form shows only the content type fields and hides the folder name that I want to create. It is a sandboxed solution (for Office 365) and I dont know why that happens...

Thanks all!