To export the tts 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:
* tts[ResourceLoader|dependencies=oojs-ui-core,oojs-ui.styles.icons-media]|tts.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.