When you add custom fields to the Task object, you can copy values from Checklist or Checklist items and display the columns (in Classic Add Checklist).
Show Custom Task Fields
After adding your custom fields to the Task Object, add them to the Field Set "Add Checklist".
The columns will be displayed on the Add Checklist page.
Note that this option is available in Classic only at this point.
When you add the custom task fields in the Page Layout, they will be displayed in the Task dialog in Classic or Lightning.
Copy Custom Task Field Values
When creating Tasks from Checklists, you can copy custom field values from the Checklist or Checklist Items. The custom fields need to be the same type and have the same name.
- Add the Custom Field "MyField__c" to the Task Object
- Add the Custom Field "MyField__c" to the Checklist Object and/or the Checklist Item Object
- When a Task is created, the MyField__c of the task is initialized with the value of the Checklist field value and/or the Checklist Item field value. (if defined on both, the Checklist Item value overwrites)