Hi Ronald,
That functionality was not available in 15.x, as far as I am aware and can find from my testing. Category names would not auto-populate in the To field. Rather, users could click on the "To" "Cc" or "Bcc" text to open a modal window where contacts from the Contacts list, GAL and categories could be added.
In 16.1.6411, we added back the shortcut to choose from a list of Contacts, GAL members and categories when composing an email message. This now behaves in the same way as it did in version 15.x.