When you're actually creating an entry via the share prompt there are no options at all unlike the browser extension (for Android at least)
So yeah, you can tag it.. if you exit your current app, find Omnivore in your launcher, open the app, find the entry you just made, then edit it, then add tags.. but that's a terrible UX.
Ah yeah, the share dialog is really basic, I ended up using Firefox and installing the full extension when I wanted to save pages (also allows you to save pages from places you're logged into)
So yeah, you can tag it.. if you exit your current app, find Omnivore in your launcher, open the app, find the entry you just made, then edit it, then add tags.. but that's a terrible UX.