Just type "#" followed by the a the name of the field, for example "#last" will show you all the merge tags that are available with "last" in the field name.
Warning: For Form URLs, ZE Portal automatically adds the Hash to the URL (do not add it again)
For example, you might create a Portal Template with a Portal Item of type "Question" and in the question you might ask: "Is ${Contacts.Mobile} your correct phone number?
ZE Portal will then replace ${Contacts.Mobile} with the value from the field with the API Name of "Mobile" in the "Contacts" module.