<ComboBox x:Name="DropDownMenu" SelectedItem="{Binding Role, Mode=TwoWay}" Style="{StaticResource DropDownMenu}" Grid.Row="1" Grid.Column="2" >
        <ComboBoxItem Content="End user" ></ComboBoxItem>
        <ComboBoxItem Content="App developer"></ComboBoxItem>
    </ComboBox>

private string role;
public string Role
    {
        get => role;
        set => Set(ref role, value);
    }