Hello Diogo,

First,my apologies for not having responded to this thread for a very long time. Though you might have figured out a solution by now, I just thought I'd put it up here .

You can probably use a combination of a checkbox + number field to achieve this using webhooks in an observer rule. You can create a checkbox named " Auto-fill user" and then create a rule as described below :

When any of these events occur -> Auto-fill user ( checkbox )  is checked

Events are performed by -> Agent

Action -> Trigger webhook 

In the webhook setup, you can use our Contact API to automatically transfer the number field data to the contact field. 

You can use {{ticket.requester.id}} in the API URL to dynamically pick the contact based on the ticket data.

Drop us a note here if you're interested in setting this up.

Cheers!