javascript fill combobox with ajax but in C# selected value is null

I have some issue with filling a combobox with javascript.

var optn = document.createElement("OPTION");
            optn.text = 'HOLA MUNDO';
            optn.value = '2';
            ComboBoxName.options.add(optn);

That is filled with ajax because its combo who are filled depending of another combo.

But in c# when i need to use

ComboBoxName.SelectedItem.Text

the SelectedItem is null

Somebody know about that? Thanks