Just in case you don't get notification that I replied to your comment on my wall, here's my reply:
Enter your e-mail address used at RingPlus, any of your phone numbers, and the MEID associated with the phone number.
Next, select "I do not agree to the Terms of Service. I would like to port out my number", then press the Continue button.
That should yield the required information for a TextNow port-in.