In CA Service Catalog & when a manager rejects a request, we need him to fill in a text area filled that will be mandatory before saving the request.
How this can be done...
Unfortunately there's no easy/supported way to do that. It's not possible to change the "required-ness" of a field programmatically. What you'd normally do here is hide or display the field depending on the situation, but in this case you're wanting to have it filled in before you perform the rejection.
Does anyone have an idea of what customization might be able to achieve something similar?
You might be able to do something similar to this thread to check if approve or reject is selected from the dropdown and hide/show the necessary field(s).