You can use a simple include, containing all code required to use the api. Just use the cdn.
Latest
<script src="ttps://cdn.jsdelivr.net/gh/sebs/etherscan-api/dist/bundle.js"></script>A specific version
<script src="https://cdn.jsdelivr.net/gh/sebs/etherscan-api@v1.3.0/dist/bundle.js"></script>