If this is a slipped tendon I don't think your going to get it down normal unless the tendon is in the right place. And if you can't bend the leg down how can you try and slip the tendon back. I am not sure using the vet wrap will help although it may save her from getting sores on her knees from walking on them .
Have you tried putting her in the cup or container and letting her legs hang down maybe the fact they are hanging may start the legs to uncurl on their own. Maybe worth trying. I wouldn't wrap unless you can get the legs to start hanging down. There are probably more on here with experience than I on this topic so if you want post in the Emergencies forum maybe someone there will have some experience.
I want to help but just not sure where to go from here.
https://anatomylearner.com/duck-anatomy/ Looks like the shank is the knee area. when looking at your other duckling where the leg bends that is what i would call the knee about where a knee is on us humans.