show a form in a popup when a button is pressed I am creating  a registration form it has multiple stages. I am able to redirect to another page with the help of a link. 
How can i get the form as a popup with a button.

 $form['adding_disease']=array( 
   '#markup'=&gt; '&lt;a href ="nojs"&gt;'.t('disease_list').'&lt;/a&gt;', 
 );


Here this snippet is written in a form with many other fields.
this link is in the middle of the form.
Another solution is to use Ctools Automodal module. This approach is less complicated to @Web-Beest answer.

Here is the example on how to use it.

function mymodule_menu() {
  $items['mymodule/form'] = array(
    'page callback' =&gt; 'drupal_get_form',
    'page arguments' =&gt; array('mymodule_form'),
    'access arguments' =&gt; array('administer mymodule'),
    'modal' =&gt; TRUE, 
  );
}


The most important line in the above code is 'modal' =&gt; TRUE,.