To export the charinsert-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:

* charinsert-core [ResourceLoader |hidden |dependencies=jquery.textSelection, user, mediawiki.storage] |charinsert-styles.css |charinsert-core.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.