First of all, the first row of table is present at index [0] so the code should be as following to access first row of data table.
dt.Rows[0]
Please find below the code to insert same value i.e. "11" in all columns of first row in datatable
foreach (DataColumn col in dt.Columns)
{
dt.Rows[0][col.ColumnName] = "11";
}
Happy Coding :)