Linking a Ticket Field to a Zendesk Customer

In our org, we allow employees at our retail sites to submit requests from iPads that are not signed into an account when members of management are not present. We have removed the ‘Your email address’ field from our ticket forms (these employees are not aware of their store's email address) and have replaced it with a ‘Store #' field with a dropdown list of our stores. I am looking for a way to run a trigger/other to have Zendesk look at the Store # they chose and match it to the store's user account in Zendesk. I've tried creating a custom field for a store with their Store # and running a trigger off that but could not find a way to do this that worked properly.

Does anyone have any ideas that could help me with this? Thanks in advance!