contact form re-direct conundrum

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Some time ago I created a contact form that went to a form processor file re-directing the signup to a page that they could download content.

When I check the form processor and the re-direct page, I can't find it in full sitemap folder.
Not sure what I have done!

if ($mail_sent == true){ 
   header("location:http://generation2.build/index.php/zxcvbnm13241");


The signup and processor works fine but I need to edit the invisible page.

Please help!

This is on the older 5.6.x.x due to 5.7 and 5.8 not working well on my webserver despite having all the correct specs.

trixiemay