Relative links to the posts on the same subsite

Hi, I have a post in which there is an image at the beginning. I would like to insert something like relative link to the same post, but it should, off course, link to post depending on site.
E.g.: on site mydomain.sk/a it should look like mydomain.sk/a/post-slug
on mydomain.sk/b … mydomain.sk/b/post-slug
Practically I just want to do the same what featured image would solve, but for some reason I don’t want to make it that way.
I googled and tried un/checking options in settings of this plugin but no luck.
So I just want to do manually what is, when broadcasting, done automatically in headline or in featured image.
Thanks in advance.

http://wordpress.org/plugins/threewp-broadcast/