How it works: based on the images, the cross bow should be facing us, it should not be behind the shield.
Step 1: open second camera POV
step 2: hold your sheild up
Step 3: charge your bow then switch to your crossbow thats already charged.
Step 4 ( the result): and thats how to get a crossbow behind a sheild
(Note: THE STUFF USED DOES NOT HAVE TO BE ENCHANTED)