[Parameter]
public EventCallback<DxSchedulerAppointmentItem> AppointmentTooltipShown { get; set; }

<DxScheduler StartDate="@DateTime.Today"
             DataStorage="@DataStorage"
             AppointmentTooltipShown="OnAppointmentTooltipShown">
    ...
</DxScheduler>

@Alert

@code {
    ...

    public string Alert { get; set; } = "";

    void OnAppointmentTooltipShown(DxSchedulerAppointmentItem args) {
        Alert = "The tooltip for the following appointment has been shown: " + args.Subject ;
    }
}