widget iframe prefill with a custom id

I have created a widget with a document, I'm able to send and compile the document by sending the widgets via link or embedding it into a webpage using the iframe url.

But is there a way to assign an unique identifier to each successful compilation?

Workflow example: I have a website where each user has its own reserved area, when there are documents that users need to sign, I prepare a new widget, in the reserved area each user will get a link to sign the document, click the url that displays a webpage with the widgets iframe; I need to associate the user-id with the signing process (or obtaining from the API an unique one-time url for the same widget), so when the user complete the signing process, I can associate this operation with the user itself and always know which users have signed a certain document.

I read that for each widget I can set a callback url to be called after every successful signing procedure, but without an initial unique identifier, I cannot associate each signing procedure to the relative user.

Is there a way to achieve what I have described here?

Thanks

Stefano.