how to choose user that saves data?

Need Help
1

I’m using cakePHP 3.8 and I can’t really find how to set database user that is saving data into database.
I thought that this

ConnectionManager::get('root');

would make it, but it doesn’t. How can I set the sending user to root?
Default one that I have has only permission to fetchData but not send it.
Btw yes root is set in config/app. I’m saving data using

        $adsTable = TableRegistry::getTableLocator()->get('ads');
    $ad = $adsTable->newEntity();
    ...
    if($adsTable->save($ad))
        echo "success";
2

What do you mean by “set database user that is saving data”? Is it different from the user that you use for reading data? If so, why? If not, why is the normal method of database configuration not viable for you?