How do I customize the add task form in Request Fulfillment?

In a SM version 9.42 solution I want to customize the drop down list in the add task form in the Task Planer for Request Fulfillment.

Actually I want to have only one selectable Task Category depending on the Request Category.

And only one Open Phase selectable (equal to the current phase of the Request).

I would prefer to have those values prefilled when the form is displayed.

I have noticed that the task title and description are inherited from the Request. The title filled in the add request task form is overwritten. Could that be prevented?