// for both forms now ( allowed Characters her for strict mode )

function my_added_login_text(){
?>
<script type="text/javascript">
	const parentElement = document.getElementById("user_login").parentNode;
	const newElement = document.createElement("p");
	newElement.className = "allowedChars";
	const newContent = document.createTextNode("Erlaubte Zeichen: a-z 0-9 <space> _ . \ - @");
	newElement.appendChild(newContent);
	parentElement.append(newElement);
</script>
<?php 
}
add_action('register_form','my_added_login_text');
add_action('login_form','my_added_login_text');