CM redirect URLs showing in Google rather than client's actual URLs

During a recent check of a client's ranking in Google for important keywords I noticed that the Campaign Monitor redirect URLs included in their campaigns is showing up in results instead of the clients actual URL for the same page. Here is an example that represents what I've seen:

A search for "chico wine shop" in Google in the US
-- The first result is the Campaign Monitor redirect URL:
-- This redirect resolves to the clients homepage: (along with all the Google Analytics tracking information)

Now, it doesn't appear that the clients ranking in Google has actually been hurt by this, but obviously, showing URL on the search results page isn't ideal and could cause customer confusion.

Does anyone know why this is happening and if there is anything I can do to fix this?

Just as one further note, I've already considered using the canonical tag to specify the URL as the primary URL to Google, but unfortunately my client's publishing platform (the excellent Squarespace) does not allow for the adding of this tag to the page.

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