Gadget export
To export the dropdown gadget, click on "Download" button, save the downloaded file, go to Special:Import on the destination wiki and upload it. Then add the following to MediaWiki:Gadgets-definition page:
* dropdown[ResourceLoader|type=general|skins=vector|dependencies=mediawiki.util,mediawiki.api|default]|dropdown.js
You must have appropriate permissions on the destination wiki (including the right to edit system messages) and import from file uploads must be enabled.