Need some help on double input value from on source

I have a form with a hidden field for id-user and I have a select field in the form that gets the text field and value field from a table “users”. The value field in the select will add the users name. Now I want to add the user_id belonging to that name to be added in the hidden field.
I don’t know how to do this, at least I can not figure this out by myself.
Can anyone help me with this please?