I have not been able to reference ASP:placeholder control from codebehind.

public void Forms_Inventory(string sql)
        {
            _ds = Utilities.Getds(sql);

            DataTable dt = new DataTable();
            dt.Columns.AddRange(new DataColumn[2] {
                    new DataColumn("Name", typeof(string)),
                    new DataColumn("Department",typeof(string)) });
            dt.Rows.Add("Brian", "IT");
            dt.Rows.Add("Ed", "Help Desk");
            dt.Rows.Add("Peter", "Engineering");
            dt.Rows.Add("Scott", "Engineering");
            StringBuilder sb = new StringBuilder();
            //Add Table Header
            sb.Append("<tr>");
            foreach (DataColumn column in dt.Columns)
            {
                sb.Append("<th style='background-color: #3366ff;border: 1px solid #000'>" + column.ColumnName + "</th>");
            }
            sb.Append("</tr>");
            //Add Table Rows
            foreach (DataRow row in dt.Rows)
            {
                sb.Append("<tr>");
                //Add Table Columns
                foreach (DataColumn column in dt.Columns)
                {
                    sb.Append("<td style='width:100px;border: 1px solid #ccc'>" + row[column.ColumnName].ToString() + "</td>");
                }
                sb.Append("</tr>");
            }

            PlaceHolder plc = new PlaceHolder();
            plc = (PlaceHolder)FindControl("plcInventory");
            plc.Controls.Add( new Literal { Text = sb.ToString() });
        }