You are looking for an 'identity column'. Something like this should work:
// Add ‘ID’ column which is the primary key
Column idColumn = new Column(table, "ID");
idColumn.DataType = DataType.Int;
idColumn.Identity = true;
idColumn.IdentitySeed = 1;
idColumn.IdentityIncrement = 1;
// Create a primary key index
Index index = new Index(table, string.Format("PK_{0}", table.Name));
index.IndexKeyType = IndexKeyType.DriPrimaryKey;
index.IndexedColumns.Add(new IndexedColumn(index, "ID"));
table.Indexes.Add(index);
// Add colums to table
table.Columns.Add(idColumn);
example code from this blog/article: http://www.christophdebaene.com/blog/2007/12/31/programmatically-creating-tables-in-sql-server-using-net-and-smo/