From my understanding — There is no way for editors to add <button> element. There is a wishlist item on creating a <button> block. But not much else from the Webflow Team on making this a possibility as shown in this thread .
I recommend you pre-design (3-4) CTA options and locations using a Link Block. Then connect a CMS Link field to the Link Block element. This will allow editors/users to input their own links as needed.