API to Retrieve All Roles for a Username

We would like to be able to call an API to be able to retrieve all the K2 roles that a user is assigned to.

We attempted to create this through a SmartObject and OData, using the URM Service > Role > GetRolesForUser. The SmartObject itself works exactly as hoped. However, because there is a required "parameter" on this method, it will not work through OData. It results in a generic "An error has occurred" error response.

Thank you