To create a custom field at runtime follow the below steps:
Open the PWR Rollups application
Click on the New button
Give a name to rollup and select Source Object from the list
Select Related Object from the list
Click the Next button and Select Rollup Type and Source Field
Click on the destination field and select "Create New Field"
Enter all details and click on save
Check the checkbox to make rollup active
Click on the Save button
*Note: Make sure you add a newly created field to the page layout of that particular object.