I hope you are well today and thank you for your question.
If you are talking about the buttons displayed in the slider shown in the top of the page then it is not icon box slider but slider revolution and you can add buttons in it as described in the following page.
Solution for this topic
You can use that button in the Icon box slider by adding following HTML code in the text mode of icon box slider as shown in the attached screenshot.
<div id="button-123" class="sh-button-container sh-button-style-5"> <span class="sh-button-text">SCHEDULE ME</span></div>