Hello again Julie.

I am sorry to disappoint you, but unfortunately this is not implemented in the plugin's built in features. However, there is a workaround for this.

For example, I created a simple manual table:

wdt_ID column is being automatically generated, and is usually hidden, but I've displayed it here for explanation purposes.

When I created a table, and added the first row, I could then modify it. So, in order to set the Datetime column to be auto-populated, I had to open the table in the database. It is being stored as wp_wpdatatable_6, and you can see its database name in Editing tab in the settings above the table:

Then, in PHPMyAdmin, I opened the table, and ran the following query:

ALTER TABLE `wp_wpdatatable_6` CHANGE COLUMN `datetime` `datetime` TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP;

After that, again in the Editing tab, above the table, I changed the "ID column for editing" from "wdt_ID" to "datetime" (as you can see in the screenshot above). Then I disabled editing for "wdt_ID" in the column settings:

datetime column, being the "ID column for editing" automatically cannot be edited, but is automatically filled.

You can also choose to hide it from view if you'd like.

I hope this helps.

Best regards.