Vulnerability in EPiServer.Forms
How I can create a link similar to existing link on "External Link" add on to Configuration?
Please provide more information. Which EPiServer version?
Are you talking about the "Manage shortcut" dialog from the Settings tab when editing a page in EPiServer 7?
What functionality is it that you want to copy - being able to link to an external URL with target options, or the validation that is used for the External Link field?
(For adding validation to properties, see http://www.david-tec.com/2012/06/EPiServer-7-Preview---Using-validation-attributes/)