The template Wiki mechanism of Overpass API works as follows: A wiki page only needs to call a link encapsulation template. That expands into the longer link to query an Overpass API custom output page. Normally, we are done here with a redirection or disambiguation page. But sometimes you want to get a custom formatted disambiguation page. How to write such a page is explained on the custom output page. We explain here how to get a template onto the Overpass API server.
To avoid that arbritrary files can be stored at the Overpass API server, it only fetches files from the OpenStreetMap wiki, and only pages beginning with "Template:". Thus, you need to store your template in the OpenStreetMap wiki and then to let Overpass API load that template.
Overpass API doesn't poll the wiki server without explicit order. To fetch a template named e.g. "Template:Foobar" from the wiki onto the Overpass API server, call
http://overpass-api.de/api/template?action=fetch&name=Template:Foobar
To check if the template has been successfully fetched or to see what the server has stored for an existing template, call
http://overpass-api.de/api/template?action=show&name=Template:Foobar