persistent_return_url. When a modal is persistent, the default close button is not shown, the only ways to close the modal is through filling out the form, hitting the return link, or hitting the escape key on the keyboard (which acts exactly like the return link).
Modal content can be anything. Content authors are free to set up single or multiple column layouts. The recommended format is text content on the left and form on the right. The return link is required, which is in its own cell under both the text content and form cells. It should not be part of the free-form authorable content, instead two required fields are required for content authors to input: return link url and return link text.