TeamF I don't think this can be achieved by CSS.
You might be able to target your own links with a CSS attribute start selector, then hide a given number of pixels off the left side of the element, but I can't think of a way to verify the link label is a raw URL and no way to account for fallback fonts that would have a different character width either.
It can be done with the PHP TextFormatter extender of Flarum though, or client-side only with javascript.