How do I create a DS image using a pre-existing OS disk?

From Konrāds Šmeļkovs @ruekonrads via Twitter:


Hello. How do I create a DS image (DS Classic VM images - premium storages) using a pre-existing OS disk? Old portal doesn't allow it and new portal doesn't seem to allow to create a "blank" like the old did where you choose existing drive/image.

You can do this with D1,D2,D3 machines from the old portal, but old portal can't spawn new DS machines, whereas the new one seems not to have the "choose existing OS disk" option.

 
Thanks,
@AzureSupport