Sarah,
I just worked this out today smoothly with Airbnb Help Centre.
I think you can do it following these steps:
1. Contact the Help centre via message requesting the RFI model 21. Ask them to transmit this request to their dedicated team;
2. Someone from the Airbnb dedicated team will send you an email with information about the model;
3. They should send you a link where you can download: a) the Airbnb tax resendecy certificate and b) the RFI model 21, which you just need to sign the VI section (name and VAT).
I hope this helps.
I worked for me and only took a couple of ours, since the request until everything was finished.
Thanks Airbnb support!
Ricardo