In order to post to a particular site, you must first have permission to post there.

The fastest way to post to a particular site is to visit the site and click the "Post" link in the upper left corner. The current site will be listed as a location by default.

If you are making a post from your own site and wish to add a different site, click "Add Sites To Audience List" during the posting process and look for the site you wish to add.

Click the "+" sign next to that site. You can add any sites for which you have permission.

When you're done, close the window by clicking the "x" in the corner.

Regardless of the method you employ, as long as the site is listed under the "To" section of the posting form, your post will be submitted to that site for publication.