There isn’t any entity of type “task” saved anywhere, from which new tasks in a project are derived. There’s a list of custom-field entities attached to the project. When you create a new task in the project, Asana says “OK, let me display and make available on that new task all of the custom fields in that list for this project.”
Unfortunately that list of custom-field entities attached to the project doesn’t contain any slots for the values of those custom fields, it’s just the definitions of the custom fields. So there’s no way for you to set a default value for a custom field that will show up in that field for new tasks.
(That last statement is not entirely true, as there are actually two ways to accomplish in effect a default value: (1) if you have a Business-level Asana subscription and it’s a dropdown-list custom field, you can use an Asana Rule to set a default value, or (2) you can use a Flowsana Rule to set a default value, and that can be done with only a Premium-level Asana subscription and with any custom field type, including text and number.)