If you want to install Relevanssi Premium as a Composer package, you can use the following URL to fetch the latest version:
https://www.relevanssi.com/update/fetch_latest_version.php?api_key=XXX
Just replace XXX with your valid API key. The URL will always return the latest version of the plugin files. If you want a specific version, you can use
https://www.relevanssi.com/update/get_version.php?api_key=XXX&version=1.15.2.1
The Composer package should look something like this:
{ "type": "package", "package": { "name": "relevanssi/relevanssi-premium", "version": "1.15.2.1", "type": "wordpress-plugin", "dist": { "type": "zip", "url": "https://www.relevanssi.com/update/get_version.php?api_key=XXX&version=1.15.2.1" }, } }