Microsoft.EntityFrameworkCore dont have method IsOptional() for PropertyBuilder

I try transfer my solution from .Net Framowork to .Net Core.When I did mapping and I dint found method IsOptional() for PropertyBuilder:

.Net Framowork:

    public class PictureMap : EntityTypeConfiguration<PictureExt>
{
    public PictureMap()
    {
        this.ToTable("Picture");
        this.HasKey(p => p.Id);
        this.Property(p => p.SeoFilename).HasMaxLength(300);
        this.Property(p => p.ExternalUrl).IsOptional();
    }
}

and its work , but use EntityFrameworkCore:look in image

where I might found IsOptional()?