ONTRAforms and the Smart Form block on ONTRApages can use the "Placeholder" field in the same way. When the ONTRApages page is a PURL, a merge field in the "Placeholder" will be populated with the contact's information. Note that if the page is not accessed as a PURL then the "Placeholder" will display the merge field itself.
More ONTRAPORT/answers on PURLS
Knowledge Base Article on PURLS