Using m.me links with ref param - facebook

Question

I'm trying to deeplink users into my facebook messenger bot, and I need to pass some data in at the same time. Supposedly this can be done by using m.me links, along with a query param.
My bot's page is https://www.facebook.com/Willy-1610141399078870/
The m.me url would be something like this. https://m.me/Willy-1610141399078870?ref=somedatagoeshere
The m.me link works to a degree on desktop, but the data isn't passed to the bot. I don't particularly need it to work on desktop.
However, when I try to open that link on ios or android, I get an error message.
Android
This user name doesn't exist. Try another code or link
iOS
Username was not found
The bot hasn't been published yet, so it won't be visible to anyone else. I need to be able to test this functionality before I publish, so I hope that's not what is causing the failure.
Has anyone else been able to use this feature?

Answer 1

It seems that the facebook-generated username can't be used for m.me links. It requires you to register an explicit username.
My page's generated name was Willy-1610141399078870, so https://m.me/Willy-1610141399078870 wasn't a valid url.
To register a username for your page, go to your page > About > Create Page #username.
Then, you can attempt to pick a username that isn't already taken. Somehow, #willy-the-test-page is already taken (?), so I got this incredibly confusing error message:
Taking the error message at face value, I decided to create another page and try again. This time, realizing that the message didn't mean what it said. Picking an available username clears the error message, and allows registration.
Just kidding! It will tell you again that the page isn't eligible.
The solution was to create a totally new page, and go through the #username creation flow again. It worked for me on the second attempt.
It's rumored that a page needs at least 25 likes to become eligible for an #username, but this isn't true in all cases.