Hi WP Dev,
of course, I should have been more detailed.
I have this test site set up & am using Firefox for testing, editing etc. and in Firefox, when I click on the button (or the text link) it shows in the bottom left of the screen, but nothing happens at all.
(screenshot : https://www.screencast.com/t/2rWdQhJMZ )
I hadn't yet got to testing it in other browsers, but after you asked, I have tested it, and I'm pleased to say it does work in Edge, IE, Chrome, Opera ... and Safari on my iPhone - so it seems the issue is with Firefox itself, and not the actual process that I set up before.
Thanks for posing the questions... so my question now becomes : does anyone have an idea why it might not work in Firefox, or how it could possibly be fixed?