You almost had it Fabians. You just need to define the Javascript function that you're calling, like this:
<div class="sqs-block button-block sqs-block-button">
<div align="left">
<a onclick="myFunction()"
class="sqs-block-button-element--large sqs-block-button-element" target="_blank">Button Text</a>
</div>
</div>
<script>
function myFunction () {
(your javascript to be triggered goes here)
}
</script>