There are three obvious routes open to you:
1) Learn to make the default value in the database table work (PREFERRED!
2) Create an AFTER INSERT trigger on the table (if 1 too hard, this is well beyond you).
3) When sending data to the table, initialize the entry for that value on the C# side and then, unless you change it, you'll be inserting that initialized (=default) value.
More help than this cannot be given because we're left basically clueless on what your doing to what table, &etc.
Also, look here