Capture Customer Signature on Tablet

I created a rich text transaction body field for an estimate and I would like my technicians to be able to capture the customers signature. How can I do that? I know it can be done because the old bundle, NextService, had it. It looks like they have a work flow or something because as soon as you load their estimate, this goes away:

The above doesn't allow me to have the customers signature from the tablet like this:

I have looked through the scripts and workflow and I don't see anything that references this transaction field. I am trying to get this to be on the invoice instead of the estimate.  For some reason this specific field isn't being pulled to the invoice form just estimate.