How can I set the default values in a ComboBox multicolumns ?

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C#

WPF

combobox

MVVM

I have a multi columns combobox. After combo population, I try to set the default values, but I get an error message

This is my codes

public ObservableCollection<Model_Sedi> Sedi { get; set; }
    private Model_Sedi _Sedi_Search;
    public Model_Sedi Sedi_Search {
	get { return _Sedi_Search; }
	set {
		_Sedi_Search = value;
		OnPropertyChanged("Sedi_Search");
	    }
        }

<ComboBox x:Name="Cmb_Sede"                    
            ItemsSource="{Binding Sedi, Mode=TwoWay}"
                          SelectedValuePath="Value"
                          SelectedItem="{Binding Sedi_Search, Mode=TwoWay}" 
                          
                          VerticalAlignment="Top" Width="189">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Text="{Binding IdSede, UpdateSourceTrigger=PropertyChanged}"></TextBlock>
                        <TextBlock Text="{Binding Sede, UpdateSourceTrigger=PropertyChanged}" Padding="10,0,0,0"></TextBlock>
                    </StackPanel>
                </DataTemplate>
            </ComboBox.ItemTemplate>
            <ComboBox.Effect>
                <DropShadowEffect Color="#FF0A0A0A" Opacity="0.6"/>
            </ComboBox.Effect>
        </ComboBox>

Now, how can I set the default values ? (like this doesn't work)

Sedi_Search.Sede = "ABC"
Sedi_Search.IdSede = 111