Roles.cs

using (var serviceScope = host.Services.CreateScope())
{
    var roleManager = serviceScope.ServiceProvider.GetRequiredService<RoleManager<IdentityRole>>();

    if (!await roleManager.RoleExistsAsync("Admin"))
    {
        var adminRole = new IdentityRole("Admin");
        await roleManager.CreateAsync(adminRole);
    }

    if (!await roleManager.RoleExistsAsync("Poster"))
    {
        var posterRole = new IdentityRole("Poster");
        await roleManager.CreateAsync(posterRole);
    }
}