I've come across many ways to solve this using Font Awesome < 5, but I can't seem to solve this in any way using Font Awesome 5.
This is how many resources point to adding a Font Awesome icon in placeholder text.
<input style="font-family:FontAwesome !important" type="text" placeholder="">
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.
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.
And then add this class to your tag.
You can also the fas class to the input.
Just bear in mind that this solution and any font-awesome style changing solutions will also change the style of any text preceding or following the unicode icon.