how to give link to export button

Question

In magento backend admin panel system->configuration->Main Website->Sales->Shipping Methods->Table Rates we have "export csv" which is downloading a file. like the same I have placed a export button my group. now how can we add action to this export button and download a file. I have placed export button in system.xml with

<export translate="label">
<label>export</label>
  <sort_order>1</sort_order>
  <show_in_default>1</show_in_default>
  <show_in_website>1</show_in_website>
  <show_in_store>1</show_in_store>
  <frontend_type>export</frontend_type>
</export>

Answer 1

The class that handles that button is Mage_Adminhtml_Block_System_Config_Form_Field_Export. and it has the url in one of the methods, so you cannot use <frontend_type>export</frontend_type> for your custom button. You have to create a new field renderer similar to Mage_Adminhtml_Block_System_Config_Form_Field_Export and add your url in there.
Try it like this:

<?php
class [Namespace]_[Module]_Block_Adminhtml_System_Config_Custombutton extends Mage_Adminhtml_Block_System_Config_Form_Field  implements Varien_Data_Form_Element_Renderer_Interface{
    protected function _getElementHtml(Varien_Data_Form_Element_Abstract $element)
    {
        $buttonBlock = Mage::app()->getLayout()->createBlock('adminhtml/widget_button');

        $params = array(
            'website' => $buttonBlock->getRequest()->getParam('website')
        );

        $data = array(
            'label'     => Mage::helper('adminhtml')->__('Do Something'),
            'onclick'   => 'setLocation(\''.Mage::helper('adminhtml')->getUrl("*/controller/action", $params) . '\' )',
            'class'     => '',
        );

        $html = $buttonBlock->setData($data)->toHtml();

        return $html;
    }
}

and your system.xml section should look like this

<export translate="label">
    <label>Some button</label>
    <sort_order>1</sort_order>
    <type>button</type>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
    <frontend_model>[module]/adminhtml_system_config_custombutton</frontend_model>
</export>