Shoutbox

This cannot be done with scripts as a script will only run once Messenger is signed in.

The only way would be to set Messenger to auto-sign in when it's opened and then use Windows Task Scheduler to schedule when you want Messenger to run, that way it will automatically run and sign in whenever you set it to

Note: You will also need to set messenger to NOT run when windows starts up, otherwise it would just run and sign-in whenever you started up the computer