Should be the same as other variables. Create the variable in PDM if it does not exist. Make sure it is the same as what is in the Solidworks template (spelling/capitalization).
The variable in PDM should look similar to:
$PRPSHEET with the attribute as the variable name (created date). Extension slddrw. Then a CustomProperty with the same attribute (created date) with extension slddrw, sldasm, sldprt...
Then you would need to edit your data card and add the new field with the same variable (created date).
It may not show for old drawings until they are re-created.