How to create custom field at runtime while creating rollup?

To create a custom field at runtime follow the below steps:

1. Open the PWR Rollups application
2. Click on the New button
   
3. Give a name to rollup and select Source Object from the list
   
4. Select Related Object from the list
   
5. Click the Next button and Select Rollup Type and Source Field
   
6. Click on the destination field and select "Create New Field" 
   
7. Enter all details and click on save
   
8. Check the checkbox to make rollup active
   
9. Click on the Save button 
   *Note: Make sure you add a newly created field to the page layout of that particular object.