Follow these steps to capture and save links params when submitting an integration :

Add an input element to your page and name the params the same as the link param that you want to capture and store.
On your link add the exact param at the end like this:
Hide your input element from the page.
When someone visited your checkout page via the above URL, the param value will be pre-added to the form element automatically. and as soon as the visitor submits the payment or a form you will find that values stored on funnel >contact tab > custom fields. or on funnel >sales tab > custom fields.

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