how do I redirect WebtoLead form in the parent window not just the Iframe?

how do I redirect WebtoLead form in the parent window not just the Iframe?

Hi Can anyone assist with the above, 

I cannot get the form subission to redirect to en entirely new page, for some reason the redirect is happening within the iframe but I need it to redirect to an entirely new page in the parent window.

The code that I currently have in place is:

<input type='text' style='display:none;' name='returnURL' value='URL' > </input>

Can anyone advise how I can get this to return in the parent window not just the Iframe?

Thanks!