Gadget export
To export the dropdown-menus-vector 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:
* dropdown-menus-vector[ResourceLoader|hidden|dependencies=mediawiki.api,mediawiki.util,user.options]|dropdown-menus-vector.js
You must have appropriate permissions on destination wiki (including the right to edit system messages) and import from file uploads must be enabled.