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In EF Core, you can use the fluent API to set a constant value for all columns that have a specific name in a database table. You can achieve this by using the HasDefaultValue method in the PropertyBuilder class.

Here's an example of how you can set a constant value for all columns named "CreatedBy" in a Blog entity:

public class Blog
{
    public int Id { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }
    public string CreatedBy { get; set; }
    public DateTime CreatedDate { get; set; }
}

public class BlogContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Blog>()
            .Property(b => b.CreatedBy)
            .HasDefaultValue("John Doe");
    }
}

In the example above, the HasDefaultValue method sets the default value for the CreatedBy column to "John Doe". This means that whenever a new Blog entity is created, the CreatedBy property will be automatically set to "John Doe" unless it is explicitly set to a different value.