A client wants to submit data to Marketing Cloud Account Engagement as well as their...

 A form handler is a feature that allows you to use your own forms to post your data to Marketing Cloud Account Engagement. A form handler is similar to a form, but instead of displaying the form on your website, you point your form to Marketing Cloud Account Engagement. This allows you to submit data to Marketing Cloud Account Engagement as well as your own database3. Form handlers are useful when you want to keep the look and feel of your existing forms, or when you want to capture data from multiple forms using one form handler