index.js

// calling a "faucet" script wavesexplorer.com/tesnet/address/3MuN7D8r19zdvSpAd1L91Gs88bcgwUFy2mn/script
// this will top up the account balance, but only once
document.querySelector(".js-invoke").addEventListener("click", function() {
    waves.invoke({
        dApp: "3MuN7D8r19zdvSpAd1L91Gs88bcgwUFy2mn",
        call: {
            function: "faucet"
        }
    }).broadcast().then(console.log)
});