Currently, the Mailchimp module only allows for entity types that have a title field marked with an entity key. The only technical reason for this, AFAIK, is that the autocomplete logic relies on this. For actually embedding it doesn't matter.
Allow all content entity types that have a field marked with a key as a label. This is implemented in the attached patch.
Closed: won't fix
2.0
Campaign Module
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Thanks for submitting this ticket — and sorry we didn’t get back to you about it sooner. We are cleaning up old issues and issue for old versions and are going to close this one. If this issue is still relevant for you, please re-open it and update the “Version” field, and include any updates to the behavior you are seeing.With a smaller issue queue, we’ll have an easier time getting back to you.