On EF core you cannot create Indexes using data annotations.But you can do it using the Fluent API. 

 protected override void OnModelCreating(ModelBuilder modelBuilder)
 {
 modelBuilder.Entity<User>()
 .HasIndex(u => u.Email)
 .IsUnique();
 }

You can read more about it here : [**Indexes**][1]


 [1]: https://learn.microsoft.com/en-us/ef/core/modeling/indexes