Remember to not use the general "Font Awesome 5" font family, you need to specifically end with the branch of icons you're working with. Here i am working the "Brands" category.
<input style="font-family:'Font Awesome 5 Brands' !important" type="text" placeholder="">
For a more sophisticated solution you could implement a class that works specifically on the placeholder text of a class like this and add that class to you input. Useful if you want a different font-family on your input values.
.useFontAwesomeFamily::-webkit-input-placeholder { /* WebKit, Blink, Edge */
font-family: "Font Awesome 5 Brands" !important;
}
.useFontAwesomeFamily:-moz-placeholder { /* Mozilla Firefox 4 to 18 */
font-family: "Font Awesome 5 Brands" !important;
}
.useFontAwesomeFamily::-moz-placeholder { /* Mozilla Firefox 19+ */
font-family: "Font Awesome 5 Brands" !important;
}
.useFontAwesomeFamily:-ms-input-placeholder { /* Internet Explorer 10-11 */
font-family: "Font Awesome 5 Brands" !important;
}
.useFontAwesomeFamily::-ms-input-placeholder { /* Microsoft Edge */
font-family: "Font Awesome 5 Brands" !important;
}
.useFontAwesomeFamily::placeholder { /* Most modern browsers */
font-family: "Font Awesome 5 Brands" !important;
}
And then add this class to your tag.
<input class="useFontAwesomeFamily" type="text" placeholder="">