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Button after form fieldset

I want to put a button after fieldset legend as shown in fig.

I have tried the code below to achieve this but that not works.

how can I do this..?

enter image description here

$fieldset = $form->addFieldset('custom_fieldset', array(
                                   'legend'=>Mage::helper('cms')->__('Custom Block'),
                                   'after_element_html' => '<button type="button" onclick="alert(\'click me!!\')">click</button>',