Add HTML attr via PHP

First page has form using method POST with action to the offer page.

<form id="value-form" method="POST" action="/offer/">

When user submits form on the first page, I want on the offer page class="add-info" to be added to the <form so then it fires up the jQuery script

<form id="formOffer" class="add-info">

Jquery

if (formOffer.hasClass('add-info')) {
        launchrocket(); 
    }

I can't figure out class tag can be added via PHP to the new form on the process page.