Maybe a checkbox would be a better option but it’d need careful explaining.
The whole point of AxCrypt v2 is to encourage online use (i.e. username and password sent to a central service). Absent AxCrypt closing down there should be no reason to not enter a real email address when prompted.
Main thing is that the service is redundant: that is, you can access your files whether AxCrypt servers are online or offline.