Trigger a Script in Custom Field on Create Screen


I am creating a request type where I added a custom field with a script in its description, as it is suggested on this page

If I add some sort of event listener in that script, is it possible to display the result of a function into that custom field while submitting a request (on the create screen) and not once the issue has been created (on the edit/view screen)?

Basically, if my event is a checkbox input being checked, can the custom field containing the script be updated with the new resulting value every time the checkbox state changes, without even submitting the request?

Thank you so much for your help!