When purchasing feeder frogs, do watch out for WC amphibian that dealers sell. Building on what Cam had previously said, WC amphibians can carry chytrid fungus which will almost means a death sentence to your frog. I'm not sure how susceptible green tree frogs are to chytrid, but WC ones may carry the disease so you might not want to risk it. Look for cheap captive frogs is what I would do.
WC bullfrogs are almost guaranteed to have chytrid on them, since they're highly resistant to chytridiomycosis (the disease chytrid cause), can carry chytrid asymptomatically, and are proven to be a vector of the disease. I always had this idea of treating WC bullfrogs with antifungal meds to cure chytrids then have them be feeder frogs, but there's no research on the potential negative impact of leftover antifungal drugs being ingested by frogs, so you can pretty much ignore what I just said.

For now maybe you can try feeding you cornuta with large crickets. Make sure you give the frog lots of cover so that it feels safe, place the enclosure in a dark & quiet room/place to reduce stress and let the frog acclimatize for a day or two. Then drop a cricket or two in the cage and leave them overnight, check the next day to see if the crickets were eaten, if not, take them out. If your Cornuta start taking the crickets it would be great news. Next step may be nigthcrawlers, a pinky or fuzzy once in a while.

If everything fails, I would start force-feeding you cornuta with samurai pacman food like what Mike had recommended.