How to generate value for a custom field based on another field, soon after value is entered in dependency field (on pressing 'Enter' or changing focus on to another field)

Hi,

I have a requirement that I need to populate two fields in the work item editor - 1) 'Search term' & 2) 'Search URL'.
Based on the value entered in the 'Search term' field, URL value is to be derived and populated in the Search URL field. Calculated value script can be used to generate Search URL but, the script would get invoked only when the user saves the work item.

But, the requirement is to have 'Search URL' generated soon after user enters value in the 'Search term' field i.e., URL should get generated without having user invoke the 'Save' work item (may be, on pressing 'Enter' or changing focus on to another field).

Please guide me how to achieve this. Since this is a critical requirement for my project, I would be thankful for your inputs in this regard.