Can be done but quite tricky but dependant on a few things.
It will also come down to the actual system installed.
Once you have your Proxmark available, scan your key and we can give you a point in the correct direction.
Your best hope is a
lf scan
and result of HiTag
If so
This will be your starting document
https://www.cs.bham.ac.uk/~garciaf/publications/Dismantling_Megamos_Crypto.pdf