How to get field value updated dynamically when another field value is changed

I have a web form which is made using xpage.Form includes combobox and editable text field.

Purpose is to update dynamically combobox value(which is selected) to text field but I dont know how to do it. Combobox is computed and it does not have onChange functions…

I would be very pleased if someone could help me with this (propably easy) problem.

With regards

Lars