Using custom fields is same in Project, Application, Version, Test Case, Test Steps. So we will explain custom field use with test case, same applicable for all entities.
Refer article How to manage Custom Fields?
Refer article How to add/edit/delete automated test case?
1. In create test case page click on custom fields dropdown to get list of available custom fields for test case.
2. Select Due Date custom field from dropdown list to add to test case page.
3. Type date in due date custom field.
4. Click on Create button to save custom field along with test case details.
5. In test case details page you can see the value of due date.
6. To delete custom field go to edit test case page, remove value for due date and click on update.