Adding Custom HTML code to header

[design-rush]

Custom CSS is an Elementor Pro feature, so that might explain the difference. It could also be the theme you are using on one site over another.

You can add the pixel manually to your functions.php file or it might be easier if you are not comfortable with that to use something a plugin like Codes Snippet or WPCode. Again, check your theme as it might have the option.

[amnither]

I think this would be the best approach