Nothing to add other than a fiddle with Carey Miller’s example stripped of all the junk this forum does to it:

https://jsfiddle.net/csssss/fv5qgev0/

The root of this is that the event page is served via one domain (ie: www.example.org) and in cleartext, and the actual donation page is served via another domain (ie: example.nationbuilder.com) and via https. Chrome (at a minimum) is blocking cookies across those domains. The workaround also fails in that it allows someone to “push through” even though the warning is displayed (clicking the “Continue” button).

Anyone considering using this be aware this can cause serious issues, including multiple charges for those that hit “Continue” regardless of the warning. They will be billed, AND they’ll see the error message. This seems to (in our experience) entice people to try again…

The official response is kind of weak. In short, it’s a security problem waiting to happen to not be able to search on all privileged access. I mean, it’s 2017, this was opened in 2012. NB is not cheap, yet I keep hitting these roadblocks where really, really basic things are just missing, known by NB, and left untouched.

If you were competing against more than NGP-VAN, you’d be out of business. I cannot count the number of sensible, rational, essential feature requests have been open for 3-4 years.