To dynamically include(load) an external javascript file to the page on certain button click
event:
html part:
<button class="getscript">Load script</button>
js part:
document.querySelector('button.getscript').addEventListener('click', function(e){
var script = document.createElement("script");
script.type = "text/javascript";
script.src = "//massage.book.me/iframe/pm_loader_v2.php?width=960&url=//massage.book.me&theme=bootstrap__square_rainbow__colored_light&layout=bootstrap__square_rainbow&timeline=modern&mode=auto&mobile_redirect=0&hidden_steps=event,unit&event=1&unit=1";
document.getElementsByTagName("head")[0].appendChild(script);
e.preventDefault();
})