Change Default Selection for Lookup field

Change Default Selection for Lookup field

I have a Lookup field that I have locked, when I unlock it - the user can select the proper Zone but I need it locked, since this may change based on user selection of another field. 

Example. 

There are 3 potential zones. User A selects the Hospital Account which is in Zone 1.
In the Zone field, there are 3 zones - and once the user Selects Zone A, only Zone 1 appears.
This works as intended, but I want to LOCK the Zone so that when user selects an Account (Hospital) the Zone autofills with Zone 1.

Hopefully this make sense, thank you!