On a standard SFDC page, user lookup fields have a drop down to the left of the input field where you can select "Standard User", "Partner User", etc (for examples, click the "Change Owner" link for any record). The drop down corresponds to the User.UserType field.
I have a VisualForce page where I have a user lookup <apex:inputField />. Of course, the UserType picklist isn't displayed for me, and when I search for a user, it's restricted to only Standard users. I need to be able to search and choose a Customer Portal user.
I just noticed that one of the URL parameters in the Lookup popup when I open it from my visualforce page is "lktp=StandardUserLookup". If I change the UserType picklist on the standard page to "Customer Portal", the parameter is "lktp=CustomerSuccessLookup". "CustomerSuccess" is the internal value for Customer Portal user types.
How can we change that parameter??