This is your 49th question, pretty much all of which can be answered with a trivial google, or a little knowledge of the classes you are playing with.
In this case, it's simple to work out what you need to do by thinking about a DataTable and what it is made up of: DataRow and DataColumn elements.
Since you want to create a new column with a preset default value, you look at the
DataColumn class[
^]
You will find it has properties, including the
DataColumn.DefaultValue Property[
^] which does exactly what you think it will.
So, why waste half an hour or longer waiting for us to tell you that, when one minute of thinking on your part and a quick bit of reading would have got you what you needed?
You are showing all the signs of being a Help Vampire - and trust me, you don't want people thinking of you that way!