To export the XFDcloser-core 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:

* XFDcloser-core [ResourceLoader |dependencies=mediawiki.util, mediawiki.api, mediawiki.Title, oojs-ui-core, oojs-ui-widgets, oojs-ui-windows, oojs-ui.styles.icons-interactions, oojs-ui.styles.icons-content, oojs-ui.styles.icons-moderation, oojs-ui.styles.icons-alerts |hidden |type=general] |XFDcloser-core.js |XFDcloser-core.css

You must have appropriate permissions on the destination wiki (including the right to edit system messages) and import from file uploads must be enabled.