If you're using Windows you should be able to start Electrum, go to the playground site, click on the link labeled "LNURL-pay," and the invoice will populate the "Send" tab in Electrum client.  This assumes that your OS is set to associate the Electrum client with links that start with bitcoin: or lightning:.

Instead of left-clicking on the link in the Playground site, you can right-click, copy link address, then past that into the "Pay To" field on the "Send" tab in the Electrum client.