Everything is still available for saving custom items - it's just been moved around a bit.Select the asset that you want to save in the main window. Open the "Content" tab and then select the sub-menu - in the screen grab below Set>Props>Custom has been selected. If needed make a new folder for the content.
With a folder opened to save the item in just click on the + sign at the bottom to add your asset - once added you can name it.