You'd have to write the event. Easiest would be editing the file - just change the value for the "flagname" field to the name of the flag, minus the tag (in the .gfx folder it should be listed as GERWeimar but you should just type in Weimar)
If you do not have the "flagname" field (which you may not if yo have the default flag) simple insert it, either above or below the "religion" field (search for flagname and see where it's located under other country tags).
When you have it under your tag it should look like this:
flagname = "weimar"