You will need to register a domain name, that is where you choose what you want to use, for example "example.com". When you register the name, you purchase it and it is yours.
Once you have a domain name, you can set up hosting. In many cases a shared hosting account will suffice. Setting up hosting means you are leasing space on a server where the files for your web site will be.
Once you have a domain name and hosting, place downloader.js in DocumentRoot.
If you don't already have HTTPS, check with your hosting company.