This is a DIY though I admit it's not obvious how you do it.
You will need to find out the name of the key you wish to bind it to, then execute the following in the console (which is brought down by the tilde key (`)):
bind key "toggle hud_althud"
except change key above to the name of the key.
If you don't know the name of the key, you can always bind it to something else in the options menu. Whatever key name appears when you bind it to something, that's its actual key name for the console.