Manual:$wgUploadMissingFileUrl

Usage
Use this setting to point wikilinks to missing files ("red file links") to a custom URL, as with  .

The URL will get   appended to it as appropriate.

If false, the value of   is used.

If that too is false, Special:Upload is used.

  was used before this setting (1.16 and below), and will be used if   is not defined (from  onwards).

For 1.17 and 1.18 you have to set  .