To reference a topic on an external site (that is a topic not from this website), you add (anywhere before the link)

  @siteName=http://myserver/mywiki/default.aspx/$$$

Then, anywhere after that you can use

  TopicName@siteName

The $$$ in the definition URL will be replaced with the topic name provided.

Here is an example: Microsoft The casing of "siteName" is arbitrary, there are no restrictions.

Also, you can create a topic called ExternalWikis containing these site=URL lines and it will be loaded for every topic, allowing for a "header file" of common external wikis.