<form action="transform_files/scripts/user_signin.php" method="POST" >
<input type="email" name="email" class="input mb-2" placeholder="Эл. почта" required>
<input type="password" name="password" class="input mb-2" placeholder="Пароль" required>
<button class="modal__button mt-3" type="submit">Войти</button>
</form>
RewriteRule user_signin transform_files/scripts/user_signin.php
<form action="/user_signin" method="POST" >
<input type="email" name="email" class="input mb-2" placeholder="Эл. почта" required>
<input type="password" name="password" class="input mb-2" placeholder="Пароль" required>
<button class="modal__button mt-3" type="submit">Войти</button>
</form>