EF Core 3

To take Ryan’s example:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<SomeTable>(entity =>
        entity.HasCheckConstraint("CK_SomeTable_SomeColumn", "[SomeColumn] >= X");
}

Docs.

EF Core 7+

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder
        .Entity<Blog>()
        .ToTable(b => b.HasCheckConstraint("CK_Blog_TooFewBits", "Id > 1023"));
}

See this for more details and changes in 7. Docs.