code: function() {
          // creation of the form element
          var form = document.createElement('form');
          form.method = 'POST';
          var cacheId = this.layer.params["_OLSALT"];
          cacheId = (cacheId ? cacheId + "_" : "") + this.bounds.toBBOX();
          form.action = OpenLayers.Util.urlAppend(this.layer.url, cacheId);
          form.target = this.id + '_iFrame';
  
          // adding all parameters in layer params as hidden fields to the html
          // form element
          var imageSize = this.layer.getImageSize(),
              params = OpenLayers.Util.getParameters(this.url),
              field;
              
          for(var par in params) {
              field = document.createElement('input');
              field.type  = 'hidden';
              field.name  = par;
              field.value = params[par];
              form.appendChild(field);
          }   
  
          return form;
      }
docs: Method: createRequestForm
  Create the html <form> element with width, height, bbox and all
  parameters specified in the layer params.
  
  Returns:
  {DOMElement} The form element which sends the HTTP-POST request to the
  WMS.