You can add a form like this:

 hook_form()
 {
   $form1["dropdown"] = array(
    '#type' => 'select',
    '#title' => t('Preview the page with themes available'),
    '#options' => $ptions,
    '#default_value' => 'defalut_value',
    '#attributes' => array('onchange' => "form.submit('dropdown')"),
 );
//Submit button:
$form1['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Submit '),
        '#attributes' => array('style' => 'display: none;'),
);

Now you can add submit functionality using hook_submit().

• I'm using D7 and getting the single quotes around dropdown encoded to ' I suppose by check_plain. How do I avoid that? – Nic Cottrell Jun 2 '13 at 16:03                                    
• Is this possible in D7 ?? – Hitesh Apr 21 '14 at 16:10                                    

Here is the simple example using '#ajax' property

$form['select'] = array(
  '#type' => 'select',
  '#title' => 'Option #1',
  '#options' => $option,
  '#ajax' => array(
    // Call function that rebuilt other field
    'callback' => 'ajax_load_field',
    'method' => 'replace',
    // div to be get replace by function output
    'wrapper' => 'chart',
    'effect' => 'fade'
  ),
);

If you generate another form field by using this drop down.

Then use AHAH for that.

$form['my_form_submit'] = array(

'#type' => 'submit',

'#value' => t('Submit'),
'#weight' => 1,
'#submit' => array('my_form_submit'),//none JS version
'#ahah' => array(
  'event' => 'click',
  'path' => 'mymodule/js',  //Your ajax function path
  'wrapper' => 'myform-wrapper',
  'method' => 'replace',
  'effect' => 'fade',
  'progress' => array(
    'type' => 'bar',
    'message' => t('Loading...')
  )      
),

While I am sure that Nicholas is way past this issue by now, this may help some who are searching for a solution.

I'm using D7 and getting the single quotes around dropdown encoded to ' I suppose by >check_plain. >How do I avoid that? – Nicholas Tolley Cottrell Jun 2 at 16:03

I just found the "Drupal" way of doing this.

Step 1, set a variable to contain dropdown by using drupal_add_js:

drupal_add_js(array('mymodule' => array('varname' => 'dropdown')), 'setting');

Step 2, add the attributes line as

'#attributes' => array('onchange' => "form.submit(Drupal.settings.mymodule.varname)"),