Support tells me that the 'Managed Metadata Picker" functionality that existed in the old add-in is not available in the Web Add-in. I'm not sure how users are supposed to know all of the available choices of a Managed Metadata field in order to use the Type-Ahead feature to fill in this metadata. A user could be saving emails to dozens of libraries each using multiple Managed Metadata columns. They need to be able to see a drop-down of the options for those fields that they can choose from. This is a huge drop in efficiency and has caused several of my users to stop using the product all-together.