We have added a functionality of defining a preferred handling agent per an aircraft (for selected airports).
In an aircraft profile, tab 'Sales', click the field 'Preferred handling' and type an airport's code, then select a handling agent. It is not possible to select more than one preferred handling agent. Once it's done, save changes.
Selected handling agent will then show automatically when adding a flight in a section OPS, or when booking a quote in 'Requests/Quotes' panel.