submitting the form in iframe and redirect the whole page

Question

i guess I have an easy question, I have not found the right answer yet thoguh. I have an iframe in my page that comes from external domain. After submiting the form which is

Answer 1

I have put target='_parent' in the iframe but I haven't done this initially in the form element. After adding target='_parent' attribute to form it started to work as expected.