By default the control isn't available in your custom classes.
You could update the existing control (but can be overwritten easily when upgrading or importing data).
- Go to the form controls module
- search for the custom table selector (or table item if you want just 1 row)
-
scroll to the scope
- select system tables (which includes your custom classes)
![Image Text](https://content.screencast.com/users/DavidTK/folders/Jing/media/8b91c2ba-b104-4c08-98e1-9791ff76db6d/2018-09-11_1542.png)
If this suits your needs you might be thinking about just cloning your own copy the form control so you're safe for upgrades and imports overwriting the default Kentico control.