Please follow below steps to create a custom product attribute and add it as WYSIWYG Editor in Product form.
Step 1 : Create custom attribute through Install Script.
Below is an example of install script.
Latest Magento version 2.3 provides the "Text Editor" Catalog Input Type for Store Owner
So please go to Store->Attributes->Product and add the New product attribute with the Catalog Input Type "Text Editor".
I hope this is helpful to you!!
In my opinion, it is totally fine that we don't send null parameters in our API, especially if it is an extension attribute. Why should we send informations that the parameter is empty, if it is optional? By default, if it wasn't passed, we can assume that it isn't present.
I would also use a custom attribute for this and would avoid using extension ...
To save the same attribute in customer, who will have to create 'sales_representative' attribute for customer.
InstallData.php/UpgradeData.php - create the eav attribute 'sales_representative'.
/* UpgradeData.php */