$Schedtask_combo1 = $main->AddCombobox( 
    -name   => "blue",
    -left   => 20, 
    -top    => 35,
    -width  => 600, 
    -height => 500,
    -style  => WS_VSCROLL | WS_VISIBLE | 3 | WS_NOTIFY,
);
$Schedtask_combo1->Add("Yellow", "Blue", "Green", "Red");
$Schedtask_combo1->Select($Schedtask_combo1->FindString("Blue"));
# or: $Schedtask_combo1->Select(1);