Using a WordPress registration form to subscribe to a list?

Is it possible to make a WordPress "new user" signup box send its email address to Sendy, if the user checks a subscribe box? I've got the checkbox showing already (see here: https://www.hilarydruxman.com/my-account/), I just don't know how to wire that into Sendy itself... or if that's even possible. Does anyone know how to make it so that when the form is submitted, check to see if the checkbox is selected and then send the email address to a list in Sendy?

Thanks!