character set in plain text email

I'm developing an application that uses the CM API to create campaigns.

It's all working nicely except for the character set in the plain text email. I would like it to be utf-8, but I don't know how to configure this on the URL where CM imports the plain text version from.

I tried just adding a html header with:

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
or
<meta http-equiv="Content-Type" content="text/plain; charset=utf-8" />

This works as far as the character set is concerned, but the whole html header is also imported as part of the plain text email...

Any ideas how to set the character set without importing any additional text?

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