you need to wait for it to bend it's head down and then you can feed it.

i think, you can reach the head if you fly with a tek jetpack or grappling hook. but that is just a thought.

you can only feed it when its bent down i tested something

i lowered its health to 0 and it does not do the animation its randomized
and you have to feed somewhere between the head and neck its weird because most of the times i failed to do it
it took me almost an hour to tame it but at least i got it