@dejalavidavolar … thanks for this. It works well.
To save others making the same error I did … the match between the page and hardcoded URL in the script is important, and means that the script will need to be edited when moved from drafts to published (or - as I discovered - won’t work as a draft, but does work once published 
).
As an aside, but related. Elsewhere on this forum fabform.io examples are provided. I’ve been unable to create an account with fabform today - no confirm your email address received (repeatedly).
I’ve used Letterbird as a Pro user for over a year and it has consistently worked well.