<form action="javascript:fn_agregar()" ; method="post" id="frm_usu">

<table class="table">

    <tr style="font-size:20px;">

        <td>
            Producto <input style="font-size:20px; font-weight:700;" class="form-control" type="text" id="txtnombreproducto" readonly required />
        </td>

        <td>
            Id <input style="font-size:20px; font-weight:700;" type="text" id="txtcodigoproducto" class="form-control" readonly required />
        </td>

        <td>
            Precio <input style="font-size:20px; font-weight:700;" type="text" id="txtprecio" class="form-control" readonly required />
        </td>

        <td>
            Cantidad <input style="font-size:20px; font-weight:700;" class="form-control" type="text" id="cantidad" value="Ingrese Cantidad" onfocus="this.value = '';" required />
        </td>

        <td>

            <div class="form-group col-md-12">

                <div class="col-md-3" style="margin-top:29px;">
                    <button class="btn btn-primary" name="btnproductos" id="btnproductos">Agregar</button>
                </div>
            </div>

        </td>

    </tr>

</table>