WPF - Binding StringFormatting Not Working

Question

Hi, I need to append a simple string to my commandparameter but doesnt work. Does StringFormat support this or am I doing anything wrong ?

  <DataTemplate x:Key="ClickableHeaderTemplate">
              <Button x:Name="btn" Content="{Binding}" Background="Transparent"
                  Command="{Binding DrilldownHeaderClicked}" 
                     Tag="{Binding RelativeSource={RelativeSource Self}, Path=Content}"
                     CommandParameter="{Binding RelativeSource={RelativeSource Self}, Path=Tag, StringFormat=somestring\{0\}}"> --- formatting doesnt work. tried without escape seq as well as in 'somesting{0}'. 
                 </Button>
            </DataTemplate>