I don’t believe this is possible today, so LMK if there’s a workaround to make this work:
Provide a CTA that only shows on web posts, not in newsletters, so I can inject in posts before I publish/send saying “Get this and other great content in our weekly newsletter”. Right now the only way I know how is to put the CTA in the header/footer theme for posts.
This could also just be a feature enhancement of the existing “Email call to action” block that inverts it to “Web call to action” block.