User Account creation and Roles assignment in Supplier Portal

Summary

Want to restrict supplier user to select the Roles

Content

Hi,

  We are in the middle of testing the Supplier Portal solution.  When we are testing.  We noticed that, when a Supplier contact is creating a new contact, he/she can check "Request User Account" which is fine, but it is also allowing the user to select the Roles which they can assign.  We have customized our Supplier portal Roles, we don't want supplier contact to select the Roles, even if he selects, he should only select our Customization roles and the delivered roles should not show up in the list.  How to achieve it?  I am attaching the document with