https://www.advancedcustomfields.com/resources/acfload_value/
add_filter('acf/load_value/name=your_field_name', 'default_value_for_your_field_name', 10, 3);
function default_value_for_your_field_name($value, $post_id, $field) {
if ($value === NULL) {
// value is NULL because it was never saved
$value = 'my default value';
}
return $value;
}