<link rel="stylesheet" href="https://stackpath.bootsrapcdn.com/bootsrap/4.3.1/css.bootsrap/min.css" integrity="sha384-gg0" > code example

Example:

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css" integrity="sha384-Vkoo8x4CGsO3+Hhxv8T/Q5PaXtkKtu6ug5TOeNV6gBiFeWPGFN9MuhOf23Q9Ifjh" crossorigin="anonymous">