<link rel="stylesheet"href="https://stackpath.bootstrapcdn.com/bootsrap/4.5.0/css/bootstrap" integrity="sha384-9aIt"> 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">