Embedding Email field into a Checkout Form

I have a campaign where I send an email to a prospect with a button linked to a Landing page.

On the landing page I have an embeded checkout form.

The issue I face is they don’t always enter the email address that is being used in the campaign and so a new record is created when they place the order. This stops the sequence continuing because it is a new entry.

I have tried the Adding URL Parameters and cannot get it to work.

Any suggestions on how to make this work.