Sharing this - hopefully it saves someone some time.
Here's a code block to create a button that opens an external website as a pop-up - just replace cnn.com with the site you want to appear in the pop-up.
<center>
<div class="sqs-block button-block sqs-block-button"><div class="sqs-block-content">
<div class="sqs-block-button-container--center preFlex flexIn" data-animation-role="button" data-alignment="center" data-button-size="medium" style="transition-timing-function: cubic-bezier(0.19, 1, 0.22, 1); transition-duration: 0.65s;">
<a href="https://cnn.com" class="sqs-block-button-element--medium sqs-block-button-element" data-initialized="true"
target="popup"
onclick="window.open('https://www.cnn.com','popup','width=600,height=600,scrollbars=no,resizable=no'); return false;">
This is CNN
</a>
</div>
</center>