Autocomplete - Controlling Dropdown Event

Autocomplete - Controlling Dropdown Event

Is there a way to control when the dropdown gets activated in this autocomplete ui part? Specifically, I would like to have it have the functionality similar to facebook, where a user can type in the box and nothing would happen until the user typed in the "@" symbol that would trigger the dropdown. 

Does this autocomplete box have this funcetionaltiy? If not, is it possible to change it to enable it?