How to horizontally center inline (Run) versus other inline in WPF

I have TextBlock

in which I put 2 Inline

( Run

s), HorizontalAlignment

for the TextBlock is set to Center, that's ok, I just want to focus the first run versus the second, here's my code:

<TextBlock HorizontalAlignment="Center" VerticalAlignment="Center" FontFamily="/Throne;component/Fonts/#Segoe UI Light" FontSize="35" FontWeight="Bold" Visibility="{Binding UserNameTextBlockVisibility}">
    <Run FontSize="25">En tant que :</Run>
    <LineBreak />
    <Run Text="{Binding UserName}" Foreground="ForestGreen"/>
</TextBlock>

      

        
        
        
      

    

Here's the result:

What I want to achieve:

I tried looking for documentation and threads on the internet, but I didn't really find a way to achieve this, how can I figure this out?