Service Catalogue - Add/Remove options in the Fields and forms rules

Hi guys,

I was wondering if you could help me understand a rule in the admin side, and whether it resolves my issue.

I'm creating a new Service catalogue and I'd like to do the following:

On Form load the field "Department" would be empty
When a user selects a "Site" from the "Site" field, the "On field edit" rule triggers

if user selects "Site A" then the "Department" field will be populated by x,y,z options
if user selects "Site B" then the "Department" field will be populated by a,b,c options.

when i edit a new service, and go to "Forms and fields rules", there is an action that is "Add options, remove options" that i thought might help, but i can't seem to get it to work.

appreciate your help! thanks.