Accessing custom fields from an HTML email template

I am trying to create a template that includes fields from the sales rep record that go beyond the list of available fields in the template editor. I know there is a way using scriptable templates to access the rest of the fields on the sales rep record - but I am missing something in my HTML code. Can someone help me out The field I want to add is a custom field on the sales rep record named custentity_nmls_id. I have a field in my template that says {salesrep.custentity_nmls_id} but is there something I need to add to the