Since it’s CMS-stored rich text the styling of the CMS element is restricted to element-type-based styling ( paragraphs, links, blockquotes, headings… etc ) rather than element-based styling where you can class and style one specific link in the rich text.
Typically you use an HTML Embed, and drop in the HTML and styling you want for your button. For example…
That style chunk could go anywhere on your blog template page. Or, you could remove it, create a DIV with a class of link-button, and style that in the designer however you like. You’ll be able to see your button design in realtime.