Change javascript code asynchronous

I use a program that allows embedding some code via a Javascript code into other websites. However, I noticed that the other websites load that code synchronous. That means if the server is slow, the other websites are waiting for the code to execute before they can load all of their own website contents. I think the best is to call that Javascript code asynchronous (like Google Analytics). How can I do that? Project is considered if you helped me that far that I could load my embed code asynchronously. Thank you