redirection page keeping data of input form

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Bids
5
Avg Bid (USD)
$34
Project Budget (USD)
$30 - $40

Project Description:
Customers click a join button using a form (with 'form action="https://xxx.blabla.com/xxxxx.pl" method="post" target="_blank"', 'input type=hidden name=.....value="........"' etc) ;
I want that, instead of being immediately led to hwere they are expected (https://xxx.blabla.com/xxxxx.pl), they first go to another, intermediate page (located in members area of another site) where they will immediately redirected to https://xxx.blabla.com/xxxxx.pl.
The reason is that before paying for site A, they must also be members of site B.
The intermediary page verifies that.
I know how to make redirections with meta tag content rtefresh, but not with a form.
Of course I could place the form in members area of site B, and just put a link to that, on site A, but this makes an additional step before people pay...

Skills required:
HTML
Hire nsvnsv
Project posted by:
nsvnsv Brazil
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