it's ave_nofalldamageaugment

so you just need to type (or copy and paste) the following after using /admin (spawns at the cursor)
/spawnitem ave_nofalldamageaugment

use /admin again afterward to be able to drain your health and energy again (and have hunger drain if not on casual)

(the 1 at the end is assumed if you don't add a number after the ID)