It seems you're misinterpreting the purpose of this token. It's not enough for it alone to be posted to Marketo, it needs to be associated with the Marketo Munchkin cookie for the current browser session on your site. (And it also needs to be immediately exchanged for the permanent customer identifier and product code, which need to be stored back to the same lead, but if you could connect it with the correct lead, that exchange can be accomplished via webhook.)
It would be simple to have the Marketplace page POST the token to the Marketo /save endpoint. But that alone wouldn't get you anywhere, because it would create a new lead in Marketo with only that one field filled in (not an Email Address or any other info). It also wouldn't have anywhere to redirect from there, because Marketplace is POSTing only one field and /save requires a redirect URL as an additional query param (retURL).
In order for this interaction to work, you'll need an intermediate gateway. It'll be very lightweight, but is necessary to tie the pieces together. It will process the Marketplace token together with the session's first-party cookies. With that information it can exchange the token, associate the lead, and redirect to the next user-facing page.