The default behaviour looks for @license or @preserve...
But many plugins and libraries are still using /*! for licensing comment blocks... 
How can I use UglifyJS2 to preserve comments starting with /*! ?
See https://github.com/mishoo/UglifyJS2#usage
Using the --comments argument, you can supply a regular expression.
uglifyjs jquery.plugin.js --comments '/^\/*!/' -o outfile.js
In case that you use this VSCode extension JS & CSS Minifier (Minify), you can configure it easily. Just add a .uglifyrc file on your project root and add the following content to your file:
  {
      "output": {
        "comments": "/^\/*!/"
    }
  }
That will only preserve your license comments