Javascript error in UI(HUB) to restrict partytype lookup to user entity

Hi Guys,

I have written a javascript to restrict partytype field to User entity

it is working fine in web but throwing error in UI, Here i am attaching JS for reference.

function SetDefaultToUser(executionContext) {
var formContext = executionContext.getFormContext();
formContext.getAttribute("optionalattendees").setLookupTypes(['systemuser']);

}

Error:

"formContext.getAttribute(...).setLookupTypes is not a function. " 

Thanks in advance