Adds a custom class to a specific WYSIWYG editor within Advanced Custom Fields.

acf-wysiwyg-body-class.php

<?php
function my_acf_admin_footer() {
	?>
	<script>
		( function( $) {
			acf.add_filter( 'wysiwyg_tinymce_settings', function( mceInit, id ) {
				if ( id === 'special-wysiwyg-field' ) {
					mceInit.body_class += ' magical-field-class';
				}
				return mceInit;
			});
		})( jQuery );
	</script>
<?php
}

add_action('acf/input/admin_footer', 'my_acf_admin_footer');