WPFのXAMLのコンボボックスで文字入力

<Window x:Class="ComboboxWithTextBox01.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:ComboboxWithTextBox01"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <StackPanel>
            <ComboBox IsEditable="True" Text="初期値">
                <ComboBoxItem Content="A" />
                <ComboBoxItem Content="B" />
                <ComboBoxItem Content="C" />
            </ComboBox>
        </StackPanel>
    </Grid>
</Window>