Magento – How to create a custom form template in magento from admin panel

Your code works (almost) correctly.
The value of the link field is not shown because it is overwritten by $form->setValues(...).
Here is how Varien_Data_Form::setValues() works

public function setValues($values)
    {
        foreach ($this->_allElements as $element) { //for each element in the form
            if (isset($values[$element->getId()])) {//if there is a value in the values for the element set the value of that element.
                $element->setValue($values[$element->getId()]);
            }
            else {//if there is no value for the element set its value to null.
                $element->setValue(null);
            }
        }
        return $this;
    }

I recommend using $form->addValues(...); this one does not set to null the missing values in the parameter passed to it.

protected function _prepareForm()
{
$form_data = new Varien_Object();

$form = new Varien_Data_Form();
$this->setForm($form);
....

if ( Mage::getSingleton('adminhtml/session')->getXyzData() )
{
    $form_data = Mage::getSingleton('adminhtml/session')->getXyzData();
    Mage::getSingleton('adminhtml/session')->setXyzData(null);
} 
else if ( Mage::registry('xyz_data') ) {
    $form_data = Mage::registry('xyz_data');  
}

$catId = Mage::getSingleton('core/session')->getCatId();   // it has value 4

if( empty($form_data->getData('category_id')) ){
    $form_data->setData('category_id', $catId);
}

$fieldset->addField('category_id', 'select', array(
  'label' => Mage::helper('lookbook')->__('Lookbook'),
  'name' => 'category_id[]',
  'values' => $categoryArray,            
 'disabled' => true
));

....

$form->setValues($form_data);

$this->setForm($form);

}