@Elli's answer can work in FontAwesome 5, but it requires using the correct font name and using the specific CSS for the version you want. For example when using FA5 Free, I could not get it to work if I included the all.css, but it worked fine if I included the solid.css:
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.8.1/css/solid.css">_x000D_
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<input type="text" placeholder=" Search" style="font-family: Arial, 'Font Awesome 5 Free'" />
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For FA5 Pro the font name is 'Font Awesome 5 Pro'