Clare,
That request from your upcoming advertiser doesn't make sense (or they're explaining what they want incorrectly).
When you send an email with Marketo we automatically add a transparent tracking pixel image so that when a recipient opens it, their email client requests the contents of this transparent tracking pixel image from Marketo's servers and we know that the corresponding email was opened. You can certainly add another platform's transparent tracking pixel image to your emails by including it in your email template or in the body content of the email itself. That way, the other platform would also be able to know when an email was opened.
Separately, we also convert any links in Marketo emails to Marketo tracking links. When a user clicks on the tracking link, it first hits Marketo's servers where we record the click, then it re-directs the user to the right location. In your case, your code is inserting an image that is prepared by the advertiser and the link to the advertiser landing page probably has some kind of token that allows them to know that someone clicked on this image in your email. When you say they want to add a tracking pixel to the URL, I'm not sure what you're asking. Do they want to add a transparent tracking pixel elsewhere in the email so that they know if it was opened, or do they want you to add something to their URL?
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