I found the solution, I just needed to do this:
<div class="login">
<h1>Login</h1>
<form action="authenticate.php" method="post">
<label for="username">
class="fas fa-user">
</label>
<input type="text" name="username" placeholder="Username" id="username" required>
<label for="password">
^__i class="fas fa-lock">
</label>
<input type="password" name="password" placeholder="Password" id="password" required>
<a href="register.html">Register</a>
<input type="submit" value="Login">
</form>
</div>
Hope this helps anyone :)
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