Populate a Joomla RSForms field with a variable

Job ID: 33205947

Budget: $10 – $30 USD

I have a website at https://integratedtravelsystems.com. We capture an affiliate ID and pass it wherever the user goes on the site. https://integratedtravelsystems.com/?aff_id=151.
That ID does not stay in the URL. I'm not sure how it is passed, but it is. I want to send that to a hidden field "Affiliate" in RSForms. I tried https://integratedtravelsystems.com/index.php/component/rsform/form/10:naughty-in-n-awlins-2022-registration?form[Affiliate]=aff_id&Itemid=426, but it sets the value as "aff_id" not the aff_id variable (151).
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