The ResearchPoint URL can only be added by creating the Raiser's Edge record via integration with ResearchPoint.  Having this field markes as required for records in The Raiser's Edge means ALL Raiser's Edge records must be created through ResearchPoint integration.

To remove the requirement in The Raiser's Edge:

Click on Config > Fields
Select Prospect from the menu
Unmark the checkbox in the ResearchPoint URL row, under the Required column