Asp.Net core MVC6 How to initially add roles in Identity 3

EDIT

Identity

In Identity RoleManager is for creating roles and UserManager is for adding users to roles. This is an example to point you in the right direction. The code below is for creating a new role Administrator

if (!roleManager.RoleExists("Administrator"))
            {
                MyIdentityRole newRole = new MyIdentityRole("Administrator", "Administrators can do something with data");
                roleManager.Create(newRole);
            }  

EDIT

Further, this is for adding a user to a role and this also an example:

 \\assuming you test if the user has been assigned to the role "Administrator" before adding them to that role

 if(RoleAdministrator == true){
           userManager.AddToRole(User.Id, "Administrator");
       }