Can I create a hidden field that is dynamically populated?

For example, right now we create RMA numbers by automatically inserting the date and time at form submission (i.e. 041920171259). Then the form is sent to the person who completed the form as well as us. This provides them with a unique RMA number to return their product and alerts us that the product is coming back.

The auto response also includes instructions on how to package and ship the product.

I don't have a JotForm account yet, but I will sign up if this can be done.

It is possible to achieve your requirement. However, merging the date and time field to generate the RMA number seems to be tricky and needs some time to check on this. But, you may also consider using the Unique ID widget on the form to generate Unique numbers for your RMA number.

https://www.jotform.com/help/79-How-to-Add-Custom-Unique-IDs-to-your-Form-Submissions

Once the form is submitted, a notification will be sent to you and an auto-responder can be sent to the form filling person. You may also customize the notification/auto-responder as per your requirement that helps to include the packaging and shipping instructions.

Cheers!