You can try just copying and pasting it, or use the icon in the toolbar for that, found here----------------
Does the new format in Xfinity e-mail not have a hyperlink tab? I can't find it anywhere.
You are probably looking at the "Cliff Notes" version of the Compose window. When you click on Compose it opens to a simpler version of the page by default and you get only part of the toolbar. Click on the double-headed arrow in the upper right corner of the window next to the "X", and it will open the full Compose page along with the complete toolbar. You will then see the icon for inserting links.