By using Givsum's Opportunity Widget you can add an "Attend" button and allow your visitors to purchase tickets and checkout from your website. Copy and paste the code provided from the link icon on your opportunity page's admin bar, or use the code below by replacing [opportunity] with the opportunity url slug.
<a href="https://www.givsum.com/opportunities/[opportunity]" class="givbtn">Attend Our Event!</a>
The "script" tag only needs to be called once and can be placed anywhere relative to your widget button / buttons.
Use a "data-style" attribute with the values "full" or "none" to make the button full-width, or remove the default styling.