Link user field with ticket field

I'd like to request the link between user field and ticket field

Ex:

I have "User Device" drop-down field in my custom ticket field. The agent or user will fill this field when they work on the ticket.
I also have "User Device" drop-down field in my custom user field. This will stick to user.
There should be a way to Auto fill the user field if the agent updates the ticket field.