At the top right of the Scoop.it editing window, you have the option to add your CTA (Call-to-action). If you click on it, you'll be able to choose the CTA you want to use for your article. It can be an image, a button, a form...
To create your library of CTAs to pull from, you need to go to "Administration" and "Assets". You can then click on "New Asset" to create a new CTA.
To help you create your own CTAs, here are some HTML examples of:
1/ An image
<a href="URL to redirect to">
<img src="URL of your image" alt="Name of your CTA" />
Note that the image you use can be hosted on your CMS backend (WordPress, etc.). You can also use any royalty-free image on the web.
2/ A button
<a href="URL to redirect to" style="font-family: Open Sans,arial,sans-serif; font-size: 17px;color: #ffffff; line-height: 50px; text-decoration: none; background-color: #4183D7; border-radius: 4px; display: block;width: 200px; text-align:center; font-weight:600">Your text here!</a>
Note that you can play with the HTML code to customize your button!