Web User Registration/Sign-up Event

Hello community,

  I am integrating Klaviyo to a proprietary web platform (web to print) and I'd like them to push a registration event to Klaviyo when a new customer registers/signs-up to the web. 

The registration is via a built-in form - not a Klaviyo form/list.

The end goal is to use that event as trigger for a welcome flow rather than using the transactional emails of the platform’s. 

Is there any documentation you can refer me to on how they should implement this new event? Alternatively, is there anything I can do with GTM and some listeners or something to achieve the desired objective?

Thanks!