Add the following snippet to a new tiddler:

\define wikipediaLink(articleName)
https://en.wikipedia.org/wiki/$articleName$
\end

<$transclude $variable="wikipediaLink" articleName={{!!wikipedia-article}}/>

Alter this snippet so the contents of the wikipedia-article field on the current tiddler can be a transclusion of a different tiddler field. (As it stands, the transclusion will render, but not as part of a link, since the transclusion within the wikipedia-article field is only seen after being returned from the wikipediaLink macro.)