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.
Therefore if you use @siteName later in this document Failed to find InterWiki 'siteName'. will point to http://myserver/mywiki/default.aspx