You can use your MB conf #, DME luggage tag number, or go to online check-in on MDE, and grab the number from the end of the URL.
I believe it has something to do with your magic band number. I believe that is what works for others. I didn't have any issues, so not certain.
Saving for later, thanks!
When in MDE (desktop version not mobile version) on the ressie page, click on "check in" as the page loads (you will see Woody while it is loading) look at the web address bar (url) at the top. Embedded address Is the numerical code (12 digit) you use"
8 min ago by