The fields available for the quick search tool offered in the right pane of the application are the project's default fields, (Email, FirstName and LastName) and the fields used as the primary key (tagged with the key icon ).

It is important to note that only mandatory fields and those composing the primary key are displayed in the quick search area tool.

 
Prerequisite Defining a field as part of the primary key can only be done when the project is in preparation and not online, since the primary key fields of an online project CANNOT be edited.
Access Path > Configuration > Project fields > Projects tab