You will need to start a chat with TextNow support to get the following information:
Account Number: (this is usually your phone number with a "1" in front of it EG: 12225551234)
Make sure that your new carrier is aware that this is porting Landline to Cellular. If your new carrier has any issues with the porting, they must email email@example.com. There is no phone number that they can call - and they have to be the ones to email.
I ported four numbers to a carrier. Two of them were successful and two were not. The two that were not, unfortunately, we were not able to complete the porting process on and eventually just gave up and got new numbers from the new carrier.
Good luck - hope it all goes well.