Official comment
Yes, you can create a Landing Page using either ONTRApages or our Legacy Landing Page editor and those will appear in the Landing Page drop down for selection. Merge fields will populate on either ONTRApages or Legacy Landing Pages, so you can have the thank you page personalized .... "Thank you [First Name]! Check your email for our free report!" would be translated to "Thank you John! Check your email for our free report!" (assuming you collect their first name on the opt in form, of course.)
You can also specify any URL as a Thank You page on a form using the "Custom URL" option. If there's an ONTRAPORT tracking script on that page we'll still track the visit just as we would a visit to our Landing Page, but merge fields will not work without some custom coding. This does allow you to have a completely branded thank you page on your own domain.