If you're referring to the "Get Started" button on the main Oxwall page, it is a custom made HTML button so you just have to inspect the code to get it.

HTML:
<div class="ow_btns_start ow_left">
   <a class="start" href="#">Label Here</a>
</div>

CSS:
.ow_btns_start{
    width: 290px;
}

.start{
    display: inline-block;
    padding: 21px 39px;
    font-size: 15px;
    color: #000;
    background: #fff;
    text-transform: uppercase;
     -webkit-transition: background 0.3s linear;
     -moz-transition: background 0.3s linear;
     -ms-transition: background 0.3s linear;
     -o-transition: background 0.3s linear;
    transition: background 0.3s linear;
}

.start:hover{
    background: #ef9411;
    transition: background 0.3s linear;
    color: #fff;
    text-decoration: none;
}

Oops, ignore my message I understand that you want the HTML button from the reply box lol.

Go to admin / settings / content settings, and uncheck the "disable custom html" box; then click save at the bottom of the page.  You will then see the "Inset html" next to the vidio icon like you see here.

Thanks! :)

HTML Computer Code..

<code>
x = 5;<br>
y = 6;<br>
z = x + y;
</code>