Answer by Tony L. for Add a column with a default value to an existing table in SQL Server

This can be done in the SSMS GUI as well. I show a default date below but the default value can be whatever, of course.

1. Put your table in design view (Right click on the table in object explorer->Design)
2. Add a column to the table (or click on the column you want to update if it already exists)
3. In Column Properties below, enter (getdate()) or abc or 0 or whatever value you want in Default Value or Binding field as pictured below: