Indeed go to the form > actions > embed code and add the lines from the example as well if they are not there:
<!--[if lte IE 8]>
<script charset="utf-8" type="text/javascript" src="//js.hsforms.net/forms/v2-legacy.js"></script>
<![endif]-->
<script charset="utf-8" type="text/javascript" src="//js.hsforms.net/forms/v2.js"></script>
<script>
hbspt.forms.create({
portalId: "#",
formId: "#",
submitButtonClass: '#',
css: ' ',
cssClass: '#',
});
</script>
Portal and form Id are already filled in.
SubmitButtonClass: add the class of the button
css: ‘’ makes sure that the default css is disabled
cssClass: add 1 class (2 won’t work is my experience, but if you succeed please let me know).