Hi Talia. Continuing along Adina's line of logic, I suggest you:
create a custom calculated date parameter called Custom Form Added
add that parm to your form(s), possibly in an Admin Only section, to hide it
save the form (so Workfront knows it is available on the form)
edit the form
set the formula to IF(ISBLANK(Custom Form Added),$$NOW,Custom Form Added)
In this manner, when a Custom Form is added (e.g. Manually, or by typing in the first piece of data on the form via a list report, thereby associating the form automatically -- cool, but a bit dangerous), the formula will kick in, see that no value exists yet, and set the value to "now" -- essentially, a do it yourself audit field. Thereafter, on subsequent edits, because the initial now value exists, it will leave it alone, preserving it.
Regards,
Doug