Been looking for a solution for the last day and cant figure out the best way of doing this

I have a user field that gets auto-updated with a user name that is already registered in the database returned using json

what I would like to figure out is how I cant autoupdate another field that is hidden with the users id

I probably need to place in in the json call but have can I get it to add to the hidden field and not added to the autocomple field for the user

any ideas
M