Allow authenticated users to update submissions Drupal 8.6.5 and Webform 5 here

I have a webform in which I have enabled Signature Pad (I mean https://github.com/szimek/signature_pad). I would like to implement a workflow such that:


An authenticated user submits the form, and an email is sent to another authenticated user with a link to the webform submission (this second user is chosen by the first user by filling a field in the form)
The second user is able to follow the link, authenticate and update the first user's submission (he adds his signature to the submission)


This is easy to accomplish when the second user is known (just a matter of access permissions), but how can I implement such a thing when he is not known/predetermined?
You might want to implement an application evaluation workflow and the Webform module includes a 'Webform Demo: Application/Evaluation System' module. 

The demo has an evaluation form attached the application form using a block.

For the email routing, you should be able to use a user entity reference token in email handlers 'To' field.