You would need to use WordPress Custom Fields:
https://wordpress.org/support/article/custom-fields/
And Hook this code into site, as per the topic i linked to:
<?php
// Get custom field value
$subtitle = get_post_meta( get_the_ID(), 'your-custom-field', true );
// Display $subtitle if not empty
if ( ! empty( $subtitle ) ) {
echo '<p class="custom-subtitle">' . $content . '</p>';
}
?>
Change the your-custom-field
slug to whatever you named your custom field.