Is it possible to customize the installation instructions globally? When clicking the Activate Duo Mobile link, I can add a custom message through the wizard but I would like to have the custom message load every time. I could not find documentation online stating that it can be set globally and I couldn't find it in the dashboard.
If you're reaching that screen by initiating activation or reactivation for a single user then you cannot customize the message and have the changes saved for global use.
However, if you go to the "Users" page in the Admin Panel, and click the "You have users who have not activated Duo Mobile. Click here to send them activation links." message at the top of the page, then once you select the users to send activation messages to you can customize that message and your changes will be retained for the next time you perform bulk activation. The process is described in more detail here.