Change output in post_system Event

In CI3 i used display_override - hook like this:

PHP Code:

function placeholders()
{
    $CI =&get_instance();
    $buffer = $CI->output->get_output();
 
    $buffer = str_replace(array('{field1}','{field2}'), array('data1','data2'), $buffer);
 
    $CI->output->set_output($buffer);
    $CI->output->_display();
}
?>

How can I access from CI4 Event's (post_system) controller to output?