Editable ComboBox with binding to value not in list

Question

I have editable combobox where not always the preferred item is in the drop-down list.

I would like to have the possibility of manually entering text in the textbox which is propagated to the string bound to SelectedValue.

Right now the string bound to SelectedValue is only updated if the entered value is on of the ones in the ComboBox items.

How do I allow custom values not available in the ComboBox list to be manually entered and properly propagated to bound value?