Since I am total newbie , I don't know if this option exists

I've seen it in the vbulletin and it's a very clever idea

There is a hide option and when I want to post a url , I write the url in this box
Now the only way that anyone could see the url is not by seeing the topic but only if he replies to it.

Can anyone tell me if there is such an option in ubbthreads (didn't find anything) or a mod ?

If there is not, it will be a very good idea if someone could make it

Thanks in advance

Not exactly what you want, but maybe the Spoiler Tag modification could work for your needs?
What it does is basically hide a text till you highlight it.