How to create a Clustered Index with Entity Framework Core

From the current EF Core documentation - Indexes section:

Data Annotations

Indexes can not be created using data annotations.

But for sure you can specify that via Fluent API (note the extension methods having ForSqlServer prefix which seem to denote SqlServer specific features):

modelBuilder.Entity<Person>()
    .HasIndex(e => e.UserName)
    .IsUnique()
    .ForSqlServerIsClustered();

Update: for EF Core 3.0+ the method is called just IsClustered:

modelBuilder.Entity<Person>()
    .HasIndex(e => e.UserName)
    .IsUnique()
    .IsClustered();

Update: Starting with EF Core 5.0, there is Index data annotation now mentioned in the Indexes documentation link, but it can't be used to specify database specific attributes like clustered (SqlServer specific), so the original answer still applies.

For EF Core 3.0+ You can now use IsClustered:

modelBuilder.Entity<Person>()
.HasIndex(e => e.UserName)
.IsUnique()
.IsClustered();

.ForSqlServerIsClustered() is now marked as obsolete.

Also be aware that if you have a Primary Key on the table you may also need to explicitly remove the clustering on it before you add the clustering on your Username:

modelBuilder.Entity<Person>()
.HasKey(e => e.PersonId)
.IsClustered(false);

modelBuilder.Entity<Person>()
.HasIndex(e => e.UserName)
.IsUnique()
.IsClustered();