Here’s an example to get data inside a message field (via code) in backend:
<?php
function my_acf_load_field( $field ) {
$field[message] = "<h3>Message</h3>";
$field[message] .= "<p>The content...</p>";
// return the field
return $field;
}
// key
add_filter('acf/load_field/key=field_5c36954abb6bf', 'my_acf_load_field');
?>