Any tips on how to get the LT code to stay with the lead as they browse the site and have it submitted with their form?

Here is the URL a lead would begin here: innovate.ieee.org/ieee-xplore-for-academia/?LT=XPLLG_AD_2018_LM_Adwords_Academic_Resources_Academic

After looking at this page, it is our goal to have them click on the Contact Us Form located on the left sidebar: innovate.ieee.org/contact-us/

However, when the lead submits the form, the LT code XPLLG_AD_2018_LM_Adwords_Academic_Resources_Academic  does not get submitted.

We have our form set up using the URL Parameter, LT, and default = Null Lead.

When we updated the form to use the Cookie Value, it was submitting a super old LT code from years back.

Any tips on how to get the LT code to stay with the lead as they browse the site and have it submitted with their form?

Thanks in advance.