What is the scope of the Create Item in another site action in regards to the destination site.
The Create Item In Another Site action will update sites in site collections that are in the same web application that the action runs in. Sites that appear in the "Select Site" popup are the sites that contain lists that can be updated with the action.
The "Enter a URL" option is meant to be used to make the action dynamic and not to enter a URL that cannot be selected using the "Select Site" option.
As web applications can have different authentication methods and the action does not have fields for credentials, this restricts the use of it crossing web applications or even farms. SharePoints Lists.asmx web service can be used to achieve a solution that goes outside the scope of this action but authentication and trust certificates still need to be in place for it to work.