Hi Bo, apologies for a delayed follow up to your questions. SoundCloud implements Twitter Card meta tags and I do see a card rendered for the URL above on web and within TweetDeck after clicking the details link in the tweet.
In order to have a Twitter Card for your subscription page, it will need to have Twitter Card meta tags in the head section of the HTML. I’m unfamiliar with MailChimp’s subscription pages such as this, but if you can edit the HTML yourself, add the tags for the card of your choice to the page source. Otherwise you will need to reach out to MailChimp.
And I assume you have control over the HTML for boporter.com. You or your web developer will need to add Twitter Card meta tags to the site for us to recognize your information and render Cards. If you have any questions, please let me know here or start a new discussion mentioning me directly.
More information about how to implement Cards for your site can be found here:
https://dev.twitter.com/docs/cards