I am not really sure what you are trying to ask us. To create a custom large text field, go to the Defects, Features, Tasks, or Incidents menu option and select "Edit Custom Fields". From there, you will be able to create a new field with a field type of "Large Text" and save the settings. This will create a new large text field in the application. From what I understand, you are trying to add a custom field to the large text field in the system which is currently not possible in the application.

Thank you,