Swift Performance has built in Varnish support. If you run autoconfig it will detect Varnish automatically, otherwise you need to enable Autopurge for Varnish on Swift Performance Settings » Caching » Varnish tab.
If you are using proxy (eg: Cloudflare) you may need to set up your server’s IP address here in order to be able to purge Varnish cache.
Usually you can use 127.0.0.1 or localhost as custom host, but it can vary, so if you are not sure, please contact to your hosting provider.