The multiselect.add() method adds an event to the current multi-selection.
DayPilot.Month.multiselect.add(e, dontRedraw)e (DayPilot.Event) - item to be added to the list of selected events
dontRedraw (boolean) - set to true if the event calendar should not be automatically updated using DayPilot.Month.multiselect.redraw()
For bulk operations, set dontRedraw to true and call DayPilot.Month.multiselect.redraw() after all changes have been made.
const e = dpm.events.find(123);
dpm.multiselect.add(e);DayPilot.Month.multiselect.redraw()
Event Selecting [doc.daypilot.org]