Here is the screen I am greeted with to start OneSignal setup for iOS. I have apple dev account and thunkable pro account, although I do not have an iOS device. Does the code they ask for (shown below) show on an iOS device that I should be signed into?
I can confirm this is 2FA. I found an old iPod in a drawer and it managed to show the pop up to allow device, and gave me a 6 digit auth code.