That does not look like a valid URL. That may be a Flicker-unique address. You need a web address.
And, rather than pasting the address into the message, it works better to insert it directly as a link. To do this, you must make your forum post in "rich formatting", using link at bottom of the message window. In the row of icons along the top are two that look like photo film, a single frame and a movie strip (multiple frames). Click on the one that is the single frame. A window will pop up, and either paste or type the address in that window and click OK.