example snippet to change magic login default message dynamically with JS

change-magic-login-shortcode-msg.php

<?php 

add_action( 'wp_footer', function () {
	?>
    <script>
		document.addEventListener('DOMContentLoaded', function () {
			var messageElement = document.querySelector('.magic-login-form-header .message');
			if (messageElement) {
				messageElement.innerText = 'Your new message here';
			}
		});
    </script>
<?php } );