How can I set value to a cutom control ?

How can I set value to custom control that are added using
Scheduler.Lightbox.AddDefaults();
Scheduler.Lightbox.Items.Insert(2, cmbplan);
Scheduler.Lightbox.Items.Insert(3, txtimagetime);
Scheduler.Lightbox.Items.Insert(4, txtimagecount);

How can I set value of above control at code behind page of c#
I am using simple asp.net application