Code-first migration: How to set default value for new property?

I am using EF6 for storing instances of the report class in my database. The database already contains data. Say I wanted to add a property to report,

public class report {
    // ... some previous properties

    // ... new property:
    public string newProperty{ get; set; }
}

Now if I go to the package-manager console and execute

add-migration Report-added-newProperty
update-database

I will get a file in the '/Migrations' folder adding a newProperty column to the table. This works fine. However, on the older entries in the database, the value for the newProperty is now an empty string. But I want it to be, e.g., "old".

So my question is: How do I set default values for new properties (of any type) in the migration script (or elsewhere)?