Applying initial filter to Users to get only specific role users

I am trying to apply initial filter to my relate field which displays all Users from users module. Here I need to show only users who are in specific role, for example I have a role called "Sales" with 15 users. So, I need only to show these 15 users in my relate field instead of all users. How can I achieve this? I have used initial filter before but it is based on field, not based on role.