Use fontawesome icons using css

Hello,
I am working on some project and I want to use font awesome icon using CSS but I am unable to use it. It works if I am using it through <i> tag.

here is the demo code which I tried.

<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.6.3/css/all.css" integrity="sha384-UHRtZLI+pbxtHCWp1t77Bi1L4ZtiqrqD80Kn4Z8NTSRyMA2Fd33n5dQ8lWUE00s/" crossorigin="anonymous">

<button><span id="prop1">button</span></button>

<span id="prop1"><i class="fas fa-ad"></i><span>

and css is

 #prop1::before{
      font-family: FontAwesome;
      content: "\f368";
    }

here is the link of the actual demo.

and I referred about it here.

please help me, where am I going wrong?

Did you remember to import fontawesome?

https://fontawesome.com/start

Yes, I imported it.
My first line of code
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.6.3/css/all.css" integrity="sha384-UHRtZLI+pbxtHCWp1t77Bi1L4ZtiqrqD80Kn4Z8NTSRyMA2Fd33n5dQ8lWUE00s/" crossorigin="anonymous">