My working trial code if anyone needs it:
function form_extras_post ($result, $event) {
if ( !is_null($_POST) ) {
$event_id = $event->post_id;
$field_name = 'organiser';
$field_value = $_POST['acf']['field_64a28d8f85a26'];
if ( ! empty( $field_value ) ) {
update_post_meta( $event_id, $field_name , $field_value );
}
}
}
add_filter( 'em_event_save', 'form_extras_post', 10, 2 );