I think you have some confusion about a normal url, a get form and and post form
a url such as index.php?page=whatever simulates the results of this get form
<form action='index.php' method='get'>
<input type='text' name='page' value='whatever' />
<input type='submit'>
</form>
If the method was post you wouldn't see the variables on the page (exception in the exception where the action is 'index.php?page=whatever' but that's not recommended).
The intention of post forms is twofoldto allow large amounts of data to be submitted, exceeding the 256 char limit on a url
to prevent the user from saving the form data as a bookmark and running again later. This is appropriate in a great many cases
If you can help us to understand the problem you are trying to resolve we may be able to help you. Is this your site you are working with, or are you trying to submit to another site from your site?
Sarah