Gadget export
To export the site gadget, click on "Download" button, save the downloaded file, go to Special:Import on destination wiki and upload it. Then add the following to MediaWiki:Gadgets-definition page:
* site[ResourceLoader|default|top|dependencies=mediawiki.legacy.wikibits,mediawiki.util,jquery.mwExtension,ext.gadget.enwp-boxes]|site.js|site.css|NavFrame.js|NavFrame.css
You must have appropriate permissions on destination wiki (including the right to edit system messages) and import from file uploads must be enabled.