custom class names do not work for your own class names

Hello

What is the use for the custom class name fields if one can not add their own class names? Only certain ones are allowed.

We are prefectly capable of wrting our own css, but no matter what class name we enter it is not added to the container of element unless you use certain apparently allowed classnames.

https://share.getcloudapp.com/WnuqNygx

What is the use of that field and what are the classnames allowed for the element as no matter what we enter the classname stays empty.

Kind regards