Hope my experience and struggles help
Without the instrument.
I find it is a very natural reaction to take a good intake of air and then if I pause it at the top of the breath and leave my mouth open it is the throat that is closing. You can push against it and engage the chest.
find and explore gently.
This is what we are trying to eliminate!!!

try a good breath in and breath out without pausing ( no throat clamping)

then try stoping flow with the tongue or lips and explore the feeling and see how the throat feels.
very easy to engage it and control flow with the throat

hope this helps. I am no expert and still struggle with this on each new higher note.
Keep at it and enjoy experimenting