How to send data like NAME[]=value via POST request and simultaneously redirect user to 3rd party server?
Try setting the name directly:

$form['order_price'] = array(
  '#type' =&gt; 'textfield',
  '#title' =&gt; t('Amount'),
  '#size' =&gt; 10,
  '#required' =&gt; TRUE,
  '#name' =&gt; 'ORDER_PRICE[]',
);